Does this sound like a virus??

Several weeks ago my PC (windows2000) started to behave oddly. When I
start it up, the screen goes blank during Windows launch. After 2 or 3
times, it will finally complete. I switched monitors and the same
problem occurs. Recently, it has started rebooting spontaneously
throughout the day. When it does this, the startup takes a very long
time on the 'applying security policy' box. I have applied the
following:
1. MS Updates
2. Fix for 'Sasser' virus (I did not have this virus)
3. anti-spyware has been run many times

Also, my email identity was trashed the other day. I'm wondering if
this was part of any virus problem, or if it was the result of the
spontaneous rebooting.

Any ideas?

Much thanks,
Jackie