Hello,
 
I put together a system about 6 months ago. Here are the specs:
 
Gigabyte n650sli motherboard
Intel E6750
MSI 8600GT 256MB  
Corsair 2GB RAM
2x WD Caviar SE16 250GB hard drive
Lite-on dvd burner
Rosewill RP500-2 ATX12V v2.01 500W Power Supply
Rosewill R5604-TBK 0.8mm SECC Screw-less Dual 120mm Fans ATX Mid Tower Computer Case
Windows XP home
 
I didn't have any problems at first, except for some occasional lockups. For the last few months the problem has been getting worse. I would get a BSOD or lockup within 20 minutes of booting up. I did notice that if I restart my computer right after booting up, I would not get any errors. I tried updating all of my drivers, but that didn't help. Please help, any suggestions would be greatly appreciated.

Check all PSU connections. Make sure all the pins are fully pushed on and not backing out of the plastic connentors. Make sure all the pins fit tight. The female connector has to be a little smaller than the male connector.  
Check HSF for proper seating and attachment.
Check Ram fully seated and locked in.
Reset bios to default and retest.
Run memtest86.  free download.

Most definitely check all of your power connectors.
 
He is also right about resetting the bios. Do you have any sort of overclock on the CPU?

Have you tried running a hard drive integrety scan?  I haven't had much luck with WD drives since the late 90s.  Now I only use hitachi desk stars.  Go to WD website and download the disk drive utilites.  Burn a bootable ISO and run the integrety test.  If it passes, it's not the Hard Drive.
 
Also, I don't have any experinece with Rosewill PSUs but a lot of people on the fourm bash them as unreliable.