Junk files on your PC don't just waste valuable disk space: they slow down performance, especially on a disk that's nearly full. On an ideal hard disk, every file would occupy its own physically contiguous sector. In practice, this state of affairs rarely, if ever, occurs most files are 'fragmented', so that the read/write head of the disk has to dash back and forth reading different sectors to load a file into memory. Similarly, if disk writers have to seek out multiple small free spaces to store a file, performance drops. Disk optimisation is a three-stage process. First you need to check each drive for errors. The procedure is the same in XP and Vista - right -click on the drive you want to check, choose Properties, turn to the Tools tab and click the 'Check now' button. Make sure at least the first option (Automatically fix ... ) is checked or nothing will be repaired.

Click the Start button and you will most probably be presented with a dialogue explaining that Windows can't check the disk while it's in use, so click the Schedule disk check button to perform the check on the next reboot. The check may take some time, so wait until you take a break before restarting. Next, perform a disk clean-up, Vista users can once again find this in Performance Information and Tools, while XP users will find it under System Tools in the Start menu.

In either case you are prompted to choose a drive and you'll be presented with a list of what can be cleaned up, such as Temporary files, old Restore points and 'lost chains' recovered by disk checking. Vista has a few more options, such as Dump files, Error report files and cleaning the Hibernation file. Having cleaned up your drive or drives, you should defragment them. Again this is available from the XP or Vista Start Menu, Accessories, System Tools, or from the Tools tab of any drive properties. Both XP and Vista let you check the drive state before defragging, and Vista lets you schedule regular defrags.