I suspect that you have a faulty driver install. You may need to uninstall your video drivers and then reinstall them... If you got them from the CD then go to the manufacturer's site and download them as the CD may be damaged.
you can also read what M$ has to say about the same issue..
I would check out each link that is shown and see if something there also applies....and The first thing I would do is reset everything back to default levels to get it fixed and then overclock from there.
r53s is on time with the warranty situation.
On the virus issue, as it is new system, it is more than likely a driver conflict that is causing you some concern To make sure just run the virus scan on the backup drive and see if there is anything there. And you should always run spyware/malware tools to keep your system clean.
On the start up issue...if you have roxio easy CD creator 5.0 then you need to uninstall it as it doesn't work well with windows xp.
Also do this:
start>my computer>properties>advanced tab>(system start-up and recovery) settings and in new window turn off auto restart in system failure/recovery and ok/apply out. Now when you shut down your system there should be a error code to say what the issues are.