This study will assess the efficacy of subsequent protease inhibitor (PI)-containing therapy in subjects who have acquired HIV-1 protease mutations whilst receiving a GW433908 (fosamprenavir)-containing regimen.

Inclusion criteria:

• Prior participation in study APV30005 AND one of the following studies: APV30001, APV30002, AZL30006, APV30003.
• Must be failing virologically (>1000 copies at two consecutive time-points) and have a screening genotype with evidence of at least one new APV-associated protease mutation: V32I (+/- I47V), I50V, I54L/M, I84V acquired since commencing treatment with GW433908.

Exclusion criteria:

• Have 2 or more of the primary resistance mutations D30N, G48V, V82A/F/T/S and L90M.
• Have a medical need for use of an non-nucleoside reductase transferase inhibitor (NNRTI).
• Have an acute CDC Class C event requiring therapeutic intervention.
• Are pregnant or lactating.
• Have any other condition which in the opinion of the investigator would preclude their participation.