Windowa XP – microsoft “asms” problem

I have a dell dimensions3000 Pentium III.
According to the microsoft website I have some kind of virus/$%#$!%$%$ problem due to an “amsa” issue which they were going to give me FREE telephone support. Well….this f$%%%$n guy was staying with me and kept trying to fix the problem but it seemed that at every attempt things got worse. I kept telling him to call MS but it was as if we were driving and he refused to get directions even though we were lost.
I can’t reboot, re-install WinXP etc. I do have an external hard drive with 80MGH. But I think it has a virus.
So, can somebody help me fix it or should i take it in to my PC guy down the street?