Strange problems with newley built system

I just built a new system with the following specs:

MSI p7n-sli platinum motherboard, Intel 8400 wolfdale processor, 2gb Gskill DDR2 pc6400 memory, and a Geforce 8800GT video card. I used my old WD 250gb hard drive in this system after reformatting it. I installed Windows XP professional as my OS.

Them computer runs very fast and cool when it is running. However, at random times (mostly when applying a patch or copying large files) it toally freezes up. The cursor is frozen on the screen and no keyboard commands whatsoever will work. If i press the restart button, it only hangs on the post screen. Then only thing that resolves this issue is to shut the system down and then turn it on. Upon rebooting, the computer runs perfectly again until another lockup.

Sometimes it will go a day or two without this happening. Sometimes it will happen 2 or 3 times a day. It seems to only do this when it is patching, loading a program, or copying files. Please let me know any diagnostics I can run to locate the culprit of this probelm. I dont want to let my time period expire to do an RMA if it is a faulty part. Thank you in advance!