A repair installation can fix some problems, but it won't clean up your PC or do much to speed it up.
If all the clean-up tutorials in the world have no effect on your PC's performance, if its reliability is at an all-time low, or you can't even boot into Windows, a complete format and reinstall is – in most cases – the solution.
Formatting your hard drive will erase everything on that drive, so it's important you back up all your data – documents, photos, email, settings and so on – before you begin. Open your backup tool and perform a manual backup to ensure the latest versions of your files are secure.
If you're unable to boot into Windows, or want the security of knowing every possible file on your computer is protected, you will need to take a complete backup – known as a 'drive image' – of your hard drive. If anything goes wrong, simply restore this backup and start again.
Again, the procedure for reinstalling Windows differs depending on whether your computer came with a recovery disc or partition, or a Windows installation disc. As with the repair installation procedure, if you're using built-in recovery options, set this procedure in motion when starting your PC – either boot from the disc or press the key when prompted to access them.
The key difference between this and a repair restore is that you want to perform a 'destructive' recovery – one which wipes all existing data from the Windows partition and returns your computer to the exact state it was in when you first switched it on.
A full reinstall
Reinstalling from the Windows installation disc is reasonably straightforward – unplug any non-essential USB devices such as printers, then pop in your Windows installation disc and restart your computer.
When prompted to press any key to boot from CD or DVD, do so, and the installation process will begin proper.
This process differs depending on whether you're reinstalling Windows XP or Windows Vista/Windows 7. For a complete guide on reinstalling Windows XP you can check out this page which should have all bases covered. Win 7 and Vista users can read on.
After Windows has loaded the basic set-up files you'll be asked to set your language: click the Time and Currency format menu to select English (United Kingdom), which is above the default setting of English (United States). Click Next followed by Install Now to continue.
Windows Vista users will be prompted to enter their product key here – do so before clicking Next. After reading the licence agreement, tick I accept the license terms and click Next again.
When prompted for the installation type, choose Custom (Advanced). A list of your hard drives and partitions will appear – select the one with your current installation and click Drive Options for more options. Click Format, read the warning – this is the point of no return – and click Yes to format the hard drive.
Sit back and wait
Once formatted, click Next and the installation proper will begin. This will take 30 minutes or so depending on the speed of your hard drive.
Once done, Windows will reboot – ignore the prompt to boot from CD or DVD this time – and you're ready to configure your new installation.
Configuration is straightforward: supply a username, a name for your computer (to identify it on your network) and a password for your account. Windows 7 users will then be prompted to enter their product key. Once done, click Use recommended settings to ensure Windows stays up to date, then verify your time zone.
The final stage involves connecting to your network: if Windows installed wireless and/or Ethernet drivers, you'll be prompted to connect to any wireless network. Once done, Windows will complete the set-up process and you'll have a fresh, uncluttered installation to work with.
Once you're back at your desktop, follow the instructions in the post-restore box to bring your computer back to working order.
Once done, the final step is to restore your data and settings using your backup tool. Whether you're using the built-in Backup and Restore tool in Windows, or using your own backup tool, you'll need to manually select your backup – in the case of the Windows Backup tool, after clicking Restore you'll be told the backup can't be found.
Don't panic: choose Select another backup (Windows 7) or Advanced restore > Files from a backup made on a different computer (Windows Vista) to locate it yourself. When you come to restore the backup, the default setting should be to restore the files to their original location, which should be the correct option in most cases.
Once your files have been restored, launch your backup tool again and create a new backup plan to ensure your files are protected in the future.
Before you reinstall: Don't start installing without this checklist
1. Get backed up
Make sure all your important files and settings are backed up – use the Windows Backup tool, or try Comodo Backup 3. Also you can take a fail-safe drive image of your entire hard drive using backup software such as Redo. You can also back up your programs and settings with our handy guide.
2. Source drivers
Make sure you can reconnect to the internet after you've reinstalled Windows: if you're reinstalling from an installation disc, you'll need to download the modem (USB connection), Ethernet (network cable) or wireless drivers from your manufacturer.
3. Verify programs
Make sure you've got any program installation discs and relevant product keys safely written down or backed up. Try Belarc Advisor if you can't find those keys, or be prepared to pay $25 for Recover Keys.
Three reasons to reinstall: Check out the benefits of taking the plunge
1. Speed up
Over time Windows gets bogged down with clutter: too many programs, leftover traces of old programs, redundant files and so on. Reinstalling Windows from scratch clears out all of this rubbish, speeding up your computer as a result.
2. Fix problems
A fresh reinstall fixes most non-hardware related problems. If your PC has reliability issues or crashes regularly, and you've ruled out hardware causes like a faulty power supply or failing hard drive, a reinstall should make it much more stable.
3. Take full control
A clean slate lets you craft Windows to your tastes and needs. Take a drive image once your 'perfect' set-up is in place, and you'll never have to reinstall again – instead, just restore the drive image and bring it up to date.