Didanosine is an effective anti-HIV drug, but it can cause stomach upset. This study tests a new form of didanosine, ddI EC, a coated pill that passes through the stomach more easily and hopefully will prevent stomach upset. The purpose of this study is to compare the effectiveness of ddI EC versus the standard form of ddI. Both forms of ddI will be given with stavudine (d4T) plus nelfinavir (NLF).