System suddenly unstable: Goes black screen & re-boots

my system was running fine previous to yesterday. i am running windows xp pro with sp2. i installed a second drive (slave) which i formatted to ntfs; added more rem from 512mb to 1 gig; added a creative sound blaster 24 bit sound card and installed firefox as my new browser. i also defragged my drives using diskeeper 10 and also did a boot defrag and adjusted my page file size. after all that, i noticed i was getting quite a bit of windows system error messages asking me to send a report for this or that. when i would launch an application from the desktop it would go black and re-boot. when i was transferring files to an external drive it would suddenly in the middle of the transfer go black and re-boot. i was running a registry cleaner application and in the middle of the scan it went black and re-booted. i cannot access any windows system information or accessories from start-up or control panel. none of my desktop shortcuts are active or linked to the applications. they do not launch from the windows start either. i have taken out all my internal cards which included my video and sound card and am running everything on board. i put back in my original memory of 512mb as well. i have also removed the second drive and am back to my original configuration. i am baffled as to what happened or what could have caused this to happen? i would appreciate any response and or help anyone can give.

It's possible that you exceeded the capacity of your Power Supply when you added that additional hard drive, additional RAM and Sound Card.
If so, everything could have gone south from that point on.
You may be able to restore the machine (without the additional hd, sound card, and additional RAM) sufficiently to be up and running again.

You can take your original Hard Drive out and install it as Slave into another computer long enough to back-up your files and data.
Then put the original (now backed-up) Hard Drive back into your machine.

Next:
Run your System File Checker
Start - Run (type) sfc /scannow
(have your OS installation CD handy to copy missing or corrupted system files for replacement)

Next
If that doesn't do the trick consider a system restore point.

Next
Failing that, consider a operating system repair re-installation.

Once you have the machine running again, consider a new power supply.

I'm not saying my analysis above is absolutely correct, but the steps I recommend are safe.

Best Regards