win 2000 won't completly boot up

I am running windows 2000 and now it won't boot up. It goes so far and then after a pause of a few minutes, a quick blue screen comes on so fast i can't read it,,then the pc starts to reboot from the beginning.
I can ge ino set up but nothing there seemed towork. Even tied safe ode wit no differenc.
I downloaded strt p and rescue disks from online to hel,,ut sil othing.
It always wants to start from C even though in set up, I changed it to A.
Could this be a virus?
How can I get started to fix the problem. I have current AVG and I guess if it is a virus,,it snuck past