NO, DON'T do that!
That will certainly give XP a royal f*cking! The windows HAL relies on the state of the ACPI toggle at install time, so if it's changed you'll probably get a nonbootable box. Anyway, if it worked before, THIS is not the problem.
I'd say, pop in the install CD and run the installer. Choose the repair option and see if that fixes things. Don't forget to connect to windows update afterwards to get patches for the "new" old files that have been overwritten from the repair.
If nothing seems to work, try booting in safe mode (with network support, if needed) and copy all the user files someplace else, another PC for example. Then format C and reinstall the OS. Annoying, but sometimes it simply has to be done. You may want to consider adding a user data partition too if there isn't one there already.