If your symptoms are being caused by entries in the registry made by a newly installed program, or an improperly removed one, restoring an earlier .reg file backup will not necessarily cure them, since this type of file will not overwrite newer registry entries. For this reason, it's a good idea to keep both .reg and hive file backups of the registry available.

Failing this, your next best option is to use the built in system restore utility to roll back the registry to a recent point. When enabled (as it is by default in Windows XP), it stores general configuration information at regular intervals, and whenever you install or remove software. By using the system restore interface, you can select one of the available snapshots and restore your registry back to its status at that point in time.

To use system restore:

Go to 'start\all programs\accessories\system tools\system restore to start the program. If you want to manually create a restore point, select the 'create a restore point' option.

To restore your PC to a specific restore point, choose the 'restore my computer to an earlier time' option, then use the calendar list of restore points to select your desired restore time.

If your PC will not boot fully into WindowsXP, you can still use system restore to a degree. If you can boot into safe mode you can use system restore to restore saved points normally. To boot into safe mode after restarting your computer, press F8 repeatedly as soon as the computer begins to boot. This will bring you to the Windows Advanced Options menu, where you can select the 'boot into safe mode' option to load Windows with a minimal set of drivers.

If you cannot even boot into safe mode, you can still hit F8 at startup and choose the 'restore last known good configuration' option which will restore Windows to the most recent system restore point, possibly saving your bacon.

Restoring a system state backup

If you made a system state backup with the XP backup utility, restoring this will revert your registry back to its state at the time of the backup.

To do this, start the backup utility and select 'restore wizard (advanced).' Now double click your backup file in the left hand pane, then place a check next to 'system state data.'

Hit 'next' and ensure that the details are correct before hitting 'finish' to restore your registry.