Windows XP has a bad habit of deleting the previous restore points at a duration of 45days.
So if you donot have the old restore point,there is no cure.
However to run a sytem restore,turn your PC on using Safe Mode and primarily if you think the error is caused by a software,you can delete it from the Programs and Features (Uninstall a program) from Control Panel.
If this doesnot work,
Run a System Restore or a Drive Format and reinstall with a newer operating system.