Firstly, check your BIOS temps. Where is your CPU at temperature wise? What is the temp of other components in the system such as the chipset? If they're higher than 60C, it should be investigated.
Otherwise, grab this: Download Hiren
Burn to CD or install to a USB drive and boot your system to it.
Run the MemTest on your system for at least two passes (it'll say on the screen how many it has done at the bottom) and if that doesn't show any errors let us know. Chances are however, that if it's not a heat issue, and the RAM tests fine, you may have a power supply that is on its way out or another transient issue with the system board. These steps will give us more insight into the system though.
Something else to check if the temps are fine and it passes the RAM tests, is to run a complete antispyware/virus scan and clean up any problems in there. You can also go to MSCONFIG (Start > Run or press the Windows key and the letter "R" at the same time then let go, then type MSCONFIG and click OK) and click the startup tab. Disable everything in there and reboot. If the problem goes away, you can re-enable items in the list and restart until you find the offensive software.