Sometimes Windows can be clogged with speed-sapping software or even malicious programs. If this happens, your best bet is to wipe the slate clean and reinstall Windows afresh.
How you achieve this depends on the company that assembled it. Custom-built desktop PCs and laptops from smaller system builders may well include a Windows installation DVD, but if your PC comes from the likes of Lenovo or HP, you'll need to follow a different process that will reset your computer to its original factory settings.
Hidden on your hard drive is something called a drive image. This is an exact copy of the Windows configuration installed on your computer when it was new, which can be copied over the damaged Windows data with the aid of some clever system recovery software.
Power up your PC and tap [F8] until a menu titled Advanced Boot Options appears. Select the 'Repair your computer' link at the top of the list and press the [Enter] key. You may find your computer's recovery software starts up straight away. If not, continue through a language and keyboard preferences screen to the point where you're required to select your usual Windows username and enter the corresponding password if necessary.
You'll now be met with a menu titled System Recovery Options. The last link is usually the one to activate the computer manufacturer's recovery software.
On a Dell machine it's likely to be labelled Dell DataSafe Restore, HP call its software Recovery Manager, and Sony's version comes under the heading VAIO Recovery Center.