Inclusion criteria:

• Males or females 2 to 18 years of age Cohorts 1A and 1B, up to one month before 6th birthday at Baseline/Day 1 Cohort 2, up to one month before 12th birthday at Baseline/Day 1 Cohort 3, up to one month before 19th birthday at Baseline/Day 1

• A female is eligible to enter and participate in this study if she is of:

  1. non-childbearing potential (i.e., physiologically incapable of becoming pregnant, including any female who is pre-menarchial); or,
  2. child-bearing potential with a negative serum pregnancy test at screen, a negative urine pregnancy test on Day 1 and who agrees to use one of the following methods of contraception (any contraception method must be used consistently and correctly, i.e., in accordance with both the product label and the instructions of a physician). Premenarchial females who develop child-bearing potential while on the study will be expected to follow one of the methods of contraception listed below.

Agreement for complete abstinence from intercourse from 2 weeks prior to administration of study drugs, throughout the study and for 2 weeks after discontinuation of all study medications. Should a female subject of childbearing potential decide to become sexually active during the course of the study, she must be counseled and be willing to use one of the contraception methods listed below:

Double barrier contraception (male condom/spermicide, male condom/diaphragm, diaphragm/spermicide) Any intrauterine device (IUD) with published data showing that the expected failure rate is less than 1% per year (not all IUDs meet this criterion) Any other method with published data showing that the lowest expected failure rate for that method is less than 1% per year.

Hormonal contraception is not recommended, due to decreased efficacy of contraception as well as increased risk of hepatic transaminase elevation (see Section 8.2).

All subjects of childbearing potential or developing child-bearing potential while participating in this study should be counseled on the practice of safe/safer sex.

• Parent or legal guardian (and subject whenever possible) has the ability to understand and provide written informed consent for the subject to participate in the trial. Verbal witnessed assent must be obtained from the subject whenever possible.
• Screening plasma HIV-1 RNA 400copies/mL.
• Subjects must meet one of the following criterion:

Antiretroviral therapy (ART)-naïve or PI-naïve subjects (defined as having received less than one week of any PI and any length of therapy with NRTIs and/or NNRTIs).

PI-experienced subjects (defined as having received greater than one week prior PI therapy with no more than three PIs). Prior RTV boosted PI therapy will be considered as only one PI as long as the RTV dose was lower than that recommended for use of RTV as an antiretroviral age

• Must have a viral load greater than or equal to 400 copies/mL.
• Must be able to take 2-3 other approved HIV medicines during the study.
• Additional qualifying criteria to be determined by the study physician.

Exclusion criteria:

• Prior history of having received AGN or FPV for >7 days.
• NNRTI use within 14 days prior to study drug administration or anticipated need for concurrent NNRTI therapy during the treatment period of the study.
• Subjects who, in the investigator's opinion, are not able to comply with the requirements of the study.
• Subject is in the initial acute phase of a CDC Clinical Category C event or infection (per 1994 classification) at Baseline. Subject may be enrolled provided they are receiving treatment for the infection, such treatment not being contraindicated with FPV, and the subjects are clinically improving at the Baseline visit.
• Presence of a malabsorption syndrome or other gastrointestinal dysfunction which might interfere with drug absorption or render the subject unable to take oral medication.
• Pregnant or lactating females.
• Presence of any serious medical condition (e.g., hemoglobinopathy, chronic anemia, a history of insulin resistance, diabetes, cardiac dysfunction, hepatitis or clinically relevant pancreatitis) which, in the opinion of the investigator, might compromise the safety of the subject.
• Grade 3 or 4 transaminase levels (ALT and/or AST) within 28 days prior to study drug administration and/or clinically relevant episodes of hepatitis within the previous 6 months.
• Any acute laboratory abnormality at screen which, in the opinion of the investigator, should preclude the subject's participation in the study of an investigational compound. If subjects are found to have an acute Grade 4 laboratory abnormality at screening, this test may be repeated once within the screening window. Any verified Grade 4 laboratory abnormality at screen would exclude a subject from study participation.
• Treatment with radiation therapy or cytotoxic chemotherapeutic agents within 28 days of study drug administration or an anticipated need for such treatment within the study period.
• Treatment with immunomodulating agents (e.g., systemic corticosteroids, interleukins, interferons) or any agent with known anti-HIV activity (e.g., hydroxyurea or foscarnet) within 28 days of study drug administration.
• Treatment with any of the following medications within 28 days prior to receiving study medication or the anticipated need during the study:

Drugs whose plasma concentration may be increased to unsafe levels when co-administered with FPV including:

Amiodarone, astemizole, bepridil, cisapride, dihydroergotamine, ergonovine, ergotamine, flecainide, halofantrine, lidocaine, lovastatin, methylergonovine, midazolam, pimozide, propafenone, quinidine, simvastatin, terfenadine, and triazolam

Drugs with the potential to significantly decrease plasma APV concentrations including:

Carbamazepine, dexamethasone, phenobarbital, primidone, rifampin, St Johns Wort.

• Treatment with other investigational drugs/therapies (note: treatments available through a Treatment Investigational New Drug [IND] or other expanded-access mechanism will be evaluated on a case-by-case basis in consultation with the sponsor) within 28 days prior to study drug administration or during the treatment period of the study.
• History of drug or other allergy which, in the opinion of the investigator, contraindicates participation in the trial or known hypersensitivity to any study medications (e.g., documented hypersensitivity to a nucleoside analogue).
• Substantial non-adherence based on history
• Must not have received AGENERASE or the study drug GW433908 for >7 days prior to study start.
• Must not have received non-nucleoside reverse transcriptase inhibitor (NNRTI) medicines within 14 days before starting study drug or anticipate the need for NNRTIs during the study.
• Cannot have had certain AIDS related illnesses within 28 days before starting study drug.
• Cannot be pregnant or breast feeding.