XP keeps rebooting since virus scan

I'm having major issues with my computer. It started when Avira was having trouble getting rid of a trojan i'd picked up from somewhere. The trojan would not dissappear but continue to reappear under a different name (started as AARKF.tmp, then AARK10.tmp, then AARK11.tmp). So i tried ending any processes that may be in use by the trojan to stop this from happening, but still no good. It was then that I restarted my computer and since then it continuously reboots just before the welcome screen. The biggest problem I have is that the computer was bought second hand and did not come with a windows XP CD (although i can borrow one from work if necessary). I've tried booting in safe mode but the same problem occurs. If anyone can help me, I'd be forever in your debt. Before I try too much though, I am going to try my HDD in my OLD computer to see what documents and such i can salvage. please help.

BennyWells