Primary: Sequentially determine the effects of three dose combinations of tipranavir (TPV) / ritonavir (RTV) (administered b.i.d.), TPV 1250 mg/RTV 100 mg vs. TPV 750 mg/RTV 100 mg vs. TPV 250 mg/RTV 200 mg on the steady-state pharmacokinetics of zidovudine, lamivudine, stavudine, didanosine, abacavir, nevirapine and efavirenz at approved doses. The three treatment groups will be enrolled sequentially starting with the highest tipranavir dosage group first and ending with the lowest tipranavir dosage group.