Over a period of time your hard disk gets cluttered up with temporary files, logs, internet files etc and can become slower
To tidy up these files and to reclaim disk space follow the instructions below for your version of Windows:-
Windows 11
Click Start > Settings > System > Storage > Temporary files
Wait for it to finish scanning, check boxes of items you wish to remove and then click Remove files
N.B. Do not select Downloads unless you want to lose all items in your Downloads folder!

Windows 10
Click Start > Settings > Storage > Temporary files
Wait for it to finish scanning and then click Remove files
N.B. Do not select Downloads unless you want to lose all items in your Downloads folder!

Windows XP, Vista, 7 and 8
Windows XP / Vista / 7 - Click Start > All Programs > Accessories > System Tools > Disk Cleanup
Windows 8 - Right mouse click on Start Menu button and choose Control Panel > System and Security > Administrative Tools > Disk Clean-up

If you have multiple drives it will first ask you which drive you wish to cleanup, this is normally C:

The tool will then scan for any files that can be removed

Once completed it will display a list of all the files to delete and the space they are taking up
Tick all the items shown in the list - except Compress Old Files (Windows XP) or System Compression (Windows 10) and then click OK to remove these files automatically
Depending on the amount of files and which items you select it may take a while to complete the cleanup