Study to characterise the effects of two dose combinations of tipranavir/ritonavir (TPV 500 mg/RTV 100 mg and TPV 750 mg/RTV 200 mg) administered BID, on the pharmacokinetics of tenofovir disoproxil fumarate as well as the effects of tenofovir disoproxil fumarate on the pharmacokinetics of tipranavir/ritonavir.