The easy way (works in 2000 and XP), and you'll want to backup first, just in case:
- Open the Device Manager.
- Expand the 'Computer' section.
- Right-click 'Advanced Configuration and Power Interface (ACPI) PC'.
- Choose properties.
- Open the 'Driver' tab. 'Update driver'.
- Select from a list.
- Choose 'Standard PC'.
You may need to have most of your driver disks and CDs handy, Windows will redetect every device and ask nicely for every driver.
There are issues with this method - Sometimes it doesn't work and renders the machine unbootable. Running setup again, hitting F5 when prompted to hit F6 and choosing 'Standard PC' should fix this. It may leave the machine highly unstable, again, running setup again should work.
Given the chance that it may just work, though, it tends to beat a re-install hands down.