How did things get this messed up? Windows has slowed to a crawl. Programs won't run. The free firewall that you installed last year won't update or uninstall itself. System Restore hasn't helped; neither have your assorted clean-up and anti-malware programs. Only one option remains: reinstall Windows and start from scratch. Yes, this is a scary, time-consuming job. Your PC may be unusable for a day or more. You could even lose all your data.

Let's face it: you would be wise to avoid this chore if it's at all possible. If someone in tech support tells you to do it, get a second opinion, and then a third. But sometimes reinstalling Windows is necessary, so here's our guide to making the process as safe and painless as possible.

Windows installation toolkit

You'll have to collect a few things before you can begin. The first is your recovery tool. If you're using the version of Windows that came preinstalled on your PC, that tool is probably in a hidden partition on the hard drive. That partition has the information necessary to restore the hard drive to its factory condition.

If your PC is a few years old, the recovery tool may instead be on one or more CDs or DVDs. Find the discs that came with your PC. The system's manual should say what kind of recovery tool came with the machine and, if it's on a partition, how to access it.

If you've upgraded Windows since you bought the machine, the upgrade disc is now your recovery tool.

If you can't find a recovery disc, and the PC has no hidden partition, contact the manufacturer to see what it can do for you. See How to install Windows without the restore CD on the next page.

After Windows installs, you'll have to reinstall all your programs. Collect the original discs or downloaded installation files, and all your licence numbers. You'll need an external hard drive with a capacity at least as large as your internal drive, and possibly a second one.

Finally, you'll need time. The best-case scenario for a Windows reinstall is a day; the worst: three or four days.

Back up everything

Things could go horribly wrong, so you need to make a backup of your hard drive and all the data stored on it.

Use cloning software to turn the external drive into an exact copy of your internal hard drive. We recommend cloning the drive using EaseUs Todo Backup. Alternatively, you can create an image backup if you prefer.

Easeus Todo Backup

Easeue Todo Backup is a program that lets you clone your hard drive. It also lets you create an image backup

Be sure to create an emergency boot disc with EaseUs or whatever program you use to do the cloning. Without that, your PC may not be able to recover from a disaster.

Having a second backup of your data wouldn't hurt - you're about to erase the original. If you don't already have another up-to-date backup, create one with your regular backup program.

Also take a look at our guide to the most suitable type of backup.