1. Go to Run type %temp% (Select all the files and delete)
Explanation: It will clear all the temperary files and some of the files will not delete, just leave it.
2. Open firefox & IE (clear all the history, cookies, passwords)
3. Start > Programs > Accessories > System tools > Disc Cleanup
(It will clear all the compressed files (check it : Really you have all of wasted memory there) you come to know that mb space were occupied of compressed files). clear all.
4. Disc cleanup will clear the recyclebin too, otherwise you can clear the files by right click on recycle bin and select empy recycle bin
5. Finally Disk Defragmentor, it takes an hour or more to complete. but sure it will give much speed for your computer. It will organize the empy space into oneside of hard disk.
6. Scan all the drives with latest updated version of anti-virus to identify the virus and clear that (AVG-Antivirus a free version for you)
7. Uninstall the unnecessary softwares from your system
8. Keep less items on your desktop. and clear the unnecessary files or move it to your folder.
9. Keep organized your folder always
10. download any registry cleaner from the net and clean the registry.
Try it first, if u not satisfy tell me. I will give more tips. But I am doing these all and my system is working fine and with good speed.
Take it to the roof of the highest building in town, and with considerable force throw it down.
If you need higher speed than that, rent an aircraft, take it up to 10.000ft, and throw it out the door.
Without adding rocket boosters that's the highest speed you're going to attain.