Reboot loops can be a bit difficult to diagnose sometimes. How comfortable are with working on the inside of the computer? If you're comfortable with doing so, I would suggest first ensuring that everything is properly seated. You will want to take proper ESD precautions, of course.
And do you have the Windows/recovery CD(s) that should've come with the computer?
The easiest solution may be to slave your drive to backup any important data and then reformat your computer.