Dell Dimension 4550 Keeps Rebooting "Part 3"

I haven't heard anything from my other post, so I thought I would make a new
one. I am actually on the computer I was having problems with. I got my
anti-virus loaded and ran, found 4 viruses. I also installed Ad-Aware, and
Spybot S&D, and ran, keep finding spyware, especially one called Admess. I
keep getting pop-ups saying that I have problems with regeristry files, and
wants me to download some regeristry repairing software.
I can't understand how this stuff was missed by the format?
.