My mom's computer had the same problem, because after about 20 minutes or so, the computer tried to go into standby mode. It never had a problem with standby until I formatted the drive and installed XP. Do you remember getting any errors during the installation?
You can also check your power management settings:
1. Right click on your desktop and choose Properties
2. Click the Screen Saver tab
3. Click the Power... button in the bottom right corner
4. Look near the bottom where it says "System standby"
5. If the drop-down box contains a value, change it to Never, then click OK. Go cook dinner, eat, cleanup, watch some TV, then come back to your computer. Is it stuck? If so, you have a different problem (obviously). :D
Hope this helps, and let us know how it goes!