Therapy is administered for 24 weeks. Patients who have an average HIV RNA value for Weeks 12 and 16 that is less than 5,000 copies or a least 1 log below their baseline value may continue their assigned study treatment for an additional 24 weeks. [AS PER AMENDMENT 3/30/98: Subjects with plasma HIV RNA greater than 5,000 copies/ml may elect to continue or discontinue study medications in the treatment extension and seek the best available treatment.] [AS PER AMENDMENT 06/11/98: The dose of adefovir dipivoxil is reduced at or after Week 16. Alternatively, patients may discontinue adefovir dipivoxil/placebo and substitute appropriate antiretroviral agent(s) or add appropriate antiretroviral agent(s) to their reduced-dose regimen. Also, at the discretion of the protocol chairperson, patients who have been on study for more than 16 weeks may substitute appropriate FDA-approved antiretroviral agent(s) for any study medication that must be discontinued because of toxicity. Addition of nonnucleoside reverse transcriptase inhibitors, protease inhibitors, or investigational agents is specifically excluded.]