Your computer can slow down due to a number of reasons. One of them could be accumulation of unnecessary files in the disk. A simple way to speed up the computer is to periodically clean the disk. There are some files, like Temporary Internet Files, that can safely be removed.
These are web pages stored in the computer's hard disk to enable quick viewing of pages previously visited. These are different from bookmarked pages, and can be safely removed. Click on Start button > All Programs > Accessories > System Tools > Disk Cleanup.
A window will open indicating the files that can be deleted, and the space that can be gained.
Another reason for slowing down of the computer is fragmentation of files and data forcing the hard disk to work more. The solution is to periodically defragment the disk.
Click on Start button > All Programs > Accessories > System Tools > Disk Defragmenter. Before the process begins, the system analyzes the data to find out if there is a need for defragmentation in the first place. The process rearranges fragmented data so the disks and drives can work more efficiently.