Sudden freezes and restarts

Hey all! I am at my wits end with my Dell Desktop. This has been ongoing for some time and it can also disappear for days at a time. Today, I have spent more than an hour (close to 2) just getting it to start properly. I start to play around then it shuts off.

Sometimes it won't go past the "Starting Windows" screen, sometimes I get a blue screen error. Sometimes it just freezes and sometimes it shuts down, or reboots! Sometimes it is perfectly fine. I have run virus scans etc and they have found nothing. I have tried restores, and I no longer have my start up discs. Any advice is appreciated! Do you think I can just purchase a new version of windows?

Stats:

Dell xPS 8000
Intel core i7 CPU 860@2.8 ghz
8.00 GB RAM (3.99 usable)
64 bit OS
Windows 7 home premium

Thanks!