PRIMARY: To evaluate the efficacy of valacyclovir hydrochloride (BW 256U87) in the prevention of cytomegalovirus (CMV) end-organ disease in HIV/CMV co-infected patients with CD4+ lymphocytes < 100 cells/mm3. To assess the impact of BW 256U87, high-dose oral acyclovir and low-dose oral acyclovir on survival.