To compare stavudine (d4T) and zidovudine (AZT) in slowing the progression of HIV disease. To compare the antiviral activity of d4T versus AZT as measured by plasma levels of p24 antigen and HIV viremia, and their relative efficacy by improvement and/or absence of adverse changes over time in laboratory parameters associated with HIV infection. To compare the safety of oral doses of d4T to AZT in patients with HIV infection.