You can test the memory by running Windows Memory Diagnostic that can be downloaded from here:
http://oca.microsoft.com/en/windiag.asp.
If memory problems are found, try re-seating the RAM in different slots. If it doesn't work, replace the defective RAM.
Miss matched Memory, using different memory types will also cause this to happen.
If your using Vista
Test Your Computer's Memory Using Windows Memory Diagnostic Tool
Using this tool will require a reboot.
Type memory into the Run box or press the Windows Key+r, and you should see the first option will say Memory Diagnostics Tool.
A dialog will pop up asking if you want to reboot and check for problems now, or check for problems the next time you restart.
If you choose to restart now, Vista will reboot into the memory diagnostic utility:
Or it can be acheived this way.
You'll have to hold down F8 on startup to get into Safe Mode, press Esc to get to the memory checker page and press TAB and then press Enter.
when you are on the screen press F1 and select Extended. Then press F10. Let it run through and let us know how you get on.
