The time to do a backup is before you need it.  Many people think of data backups, but people rarely ever think about doing a system restore point.  If your system is running pretty good right now, you should do a system restore point.  The system restore point will save you much grief in the future.  When something happens to your system functionality you can restore back to the last known good system restore point.

Don’t just sit there, create a system restore point now before you need it.

Windows XP directions

Windows 7/Vista directions