PC keeps restarting - Help!

About a week ago, I added 1GB of RAM (now 2GB) and a new WD 320 GB HD to my 5 or 6 year old Sony VAIO PCV-RS430G, and reinstalled Windows XP on it. Just this morning when switching from my daughter's desktop to mine, it froze, and I had to power it off and turn it back on. Now no matter who's desktop I go to, just as about everything is finished loading, the PC restarts. Any ideas?

Greg

Tap F8 at boot, select "don't automatically restart...", next time it blue-screens, post the 0x000000?? error & any offending program.

To get the error message from the blue screen (a.k.a. the BSOD --
Blue Screen of Death), you'll need to turn off WinXP's automatic
reboot "feature:" Right-click My Computer > Properties > Advanced >
Startup and Recovery > Uncheck "Automatically restart." The next time
your PC crashes, you'll be left with a blue screen full of memory
addresses and other seeming gibberish. The top 2 or 3 lines of this
screen will contain information that can help solve your problem.
Carefully write down these lines and post the exact text back here.
Hopefully, one of us will be able to provide some insight.