1. Free up disk space 
By freeing disk space, you can improve the performance of your computer. The Disk Cleanup tool helps you free up space on your hard disk. Tip: Typically, temporary Internet files take the most amount of space because the browser caches each page you visit for faster access later.

2. Speed up access to data
Disk fragmentation slows the overall performance of your system. When files are fragmented, the computer must search the hard disk when the file is opened to piece it back together.

 3. Detect and repair disk errors
In addition to running Disk Cleanup and Disk Defragmenter to optimize the performance of your computer, you can check the integrity of the files stored on your hard disk by running the Error Checking utility.

4. Protect your computer against spyware
Spyware collects personal information without letting you know and without asking for permission. To combat spyware, you might want to consider using Microsoft Windows Defender, which is included in Windows Vista, and is available as a free download for Microsoft XP SP2. Alternatively, there are other free anti-spyware software programs available.

 5. Learn all about ReadyBoost  If you're using Windows Vista, you can use ReadyBoost to speed up your system. A new concept in adding memory to a system, it allows you to use non-volatile flash memory—like a USB flash drive or a memory card—to improve performance without having to add additional memory.