It could be any number of things, including overheating or poor power. But these are only two possible causes out of dozens. Frugality would suggest pursuing the free/laborious possibilities first though- then finally adding case fans (or pointing a floor fan directly at the open case) or purchasing a better power supply.
I'm also assuming you've already tried installing the motherboard chipset (and AGP chipset, if applicable though not usually applicable with Intels unless you have a VIA or SiS chipset generally). If not, do this first and try.
Other possible causes (aside from chipset/agp drivers) can be incorrectly installed or conflicting drivers. I'd recommend the following "CLEAN INSTALL" steps prior to going much further:
1) Hit
www.drivercleaner.net and get Driver Cleaner Pro. Install it, run the Live Update, but DO NOT RUN IT. Close it after running Live Update.
2) Hit
www.ati.com and download the 5.7 Catalyst drivers (5.8's do have a couple problems, but they may not effect you. I'd say START with the 5.7's for best results, then repeat the process again if you wish to try the 5.8s). Get the Control Panel version for simplicity. The direct link is:
http://www2.ati.com/drivers/5-7_xp-2k_dd_cp_wdm_24085.exe
Download and SAVE THESE TO YOUR DESKTOP (i.e. don't install them yet).
3) Now, hit the Control Panel: Add/Remove Programs. Find the programs (you may not have all of them) and uninstall them in this order. Do NOT restart until the FINAL uninstall. (i.e. click No or "I'll restart Later" buttons after each):
ATI Multimedia Center
ATI DVD Decoder
ATI Control Panel (or ATI Catalyst Control Center)
ATI Display Driver
ATI Software Uninstall Utility.
4) After the final install, YANK THE NETWORK CABLE so as there is NO BROADBAND or INTERNET ACCESS. Leave it disconnected until much later.
5) Go ahead and restart, but this time boot into Safe Mode (Press and hold F8 after your BIOS/memory test screens and select "Safe Mode" at the top, then Windows XP on the second screen).
6) Log-in as Administrator in Safe Mode.
7) Browse your computer to C:\Program Files\Driver Cleaner Pro and double click on the program (there will be no shortcut desktop or start menu for administrator).
8) Run Driver Cleaner Pro- select "Multiple Cleaning Filters" and select ALL the ATI ones (add them to the right box). Don't worry about one's you don't have. They don't matter and wont hurt to add anyways. If you had another brand videocard prior, I'd also recommend adding their filters (example: NVidia, SiS, S3, etc.etc.) Once all are selected, run the cleaner.
9) Once the cleaning is completed, restart the computer but let it boot up in Normal mode (don't interrupt normal boot process).
10) On desktop, you may get two Add New Hardware pop-ups. Just hit Cancel to them both.
11) Run the 5.7 Catalyst driver, express install on your desktop from #2 above.
12) Once this is completed, go ahead and select OK to Restart your computer.
13) Let your computer restart and settle to the desktop for several minutes, you can now reconnect your broadband/internet (and reboot if needed, albeit highly unlikely).
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Test your games. These steps above almost 99.9% of the time assure properly installed, clean drivers with no remnants.
