• Proportions of subjects with plasma HIV-1 RNA levels <400 copies/mL at each study visit. Change from baseline in percentage of CD4+ lymphocytes. Plasma GW433908 concentrations. Incidence of viral resistance. Subject adherence. [ Time Frame: 48 Weeks ]
• Change from Baseline in the percentage of CD4+ lymphocytes at each study visit (absolute values and time-averaged) Plasma FPV concentrations Plasma RTV AUC,ss, Cmax,ss and C,ss following multiple dose administration of FPV/RTV BID
• Relationship of steady-state plasma APV PK parameters to changes in plasma HIV-1 RNA concentrations, CD4+ percentages and/or the occurrence of adverse events
• Subject adherence and parent/guardian perceptions of study medications Incidence of viral resistance (where permissible by blood volumes)

This is a 48-week study to evaluate the safety, tolerability, pharmacokinetics, and antiviral activity of an investigational regimen including FDA approved HIV drugs in HIV-infected pediatric subjects, ages 4 weeks to < 2 years old.

• Drug: GW433908
• Drug: ritonavir

Inclusion Criteria:

• Male or female 4 weeks to <2 years of age. Cohort 1 (6 months - <2 years): Subjects must be <2 years of age at the Week 2 visit therefore the maximum age at screening is 22 months.

Cohort 2 (4 weeks - <6 months): Subjects must be <6 months of age at the Week 2 visit, therefore the maximum age at screening is 4 months for entry into this cohort.

• Parent or legal guardian is willing and able to provide written informed consent for the subject to participate in the trial.
• Screening plasma HIV-1 RNA level 400copies/mL.
• Subjects who, in the investigator's opinion, and following viral resistance testing if conducted, are able to construct an active NRTI backbone regimen consisting of 2 NRTIs.
• Subjects must meet one of the following criteria:

Therapy-naïve or PI-naïve subjects (defined as having received less than one week of any PI).

PI-experienced subjects defined as having prior experience 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 agent.

• Must be able to construct an active 2 NRTI (nucleoside reverse transcriptase inhibitor) backbone regimen.

Exclusion Criteria:

• Prior history of having received AGN.
• NNRTI therapy within 14 days prior to study drug administration (single or multiple dose) or anticipated need for concurrent NNRTI therapy during the study period.
• PI therapy within 5 days prior to study drug administration (applicable only for subjects undergoing single dose visits)
• Subjects and/or parents/legal guardians 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 infections, such treatment not being contraindicated with FPV, and 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.
• Presence of any serious medical condition (e.g., hemoglobinopathy, chronic anemia, diabetes, cardiac dysfunction, hepatitis, or clinically relevant pancreatitis) which, in the opinion of the investigator, might compromise the safety of the subject.
• 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 would exclude a subject from study participation.
• Grade 3 or higher (>10x ULN) serum aminotransferase levels (alanine aminotransferase, ALT and/or aspartate aminotransferase, AST) within 28 days prior to study drug administration and / or clinically relevant hepatitis within the previous 6 months.
• 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:

Amiodarone, astemizole, bepridil, bupropion, cisapride, clorazepate, clozapine, diazepam, dihydroergotamine, encainide, ergonovine, ergotamine, estazolam, flecainide, flurazepam, lovastatin, meperidine, methylergonovine, midazolam, pimozide, piroxicam, propafenone, propoxyphene, quinidine, simvastatin, terfenadine, and triazolam (these drugs have been excluded for safety reasons).

Carbamazepine, dexamethasone, phenobarbital, primidone, rifampin, St Johns Wort, (these drugs have been excluded because they have the potential to decrease plasma protease inhibitor concentrations).

• Treatment with other investigational drugs/therapies within 28 days prior to receiving study medication (note: treatments available through a Treatment IND or other expanded-access mechanism will be evaluated on a case-by-case basis in consultation with the sponsor).
• 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).
• Prior history of having received the drug AGENERASE.
• Non-nucleoside reverse transcriptase inhibitor (NNRTI) therapy within 14 days prior to study Day 1 or anticipated need for concurrent NNRTI therapy during the study period.
• PI (protease inhibitor) therapy within 5 days prior to study drug administration.
• Have not had an AIDS defining illness (acute CDC Category C event) within 28 days of screen.
• Other inclusion or exclusion criteria to be determined by the physician.