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1. Go through your Add/Remove program list and remove any applicationthat you don't use.
Start > Settings* > Control Panel > Add/Remove Programs
*if using classic mode
2. Stop unnecessary programs from starting when Windows starts.
Go to Start > Run and type "msconfig" and click "OK". You will get
the "system configuration utility".
Use this page for instructions on how to narrow down which ones to stop.
http://netsquirrel.com/msconfig/
3. Make sure that you are running a good antivirus / anti-spyware program.
There are plenty of free programs out there for spyware, but I suggest
purchasing a program that does both from one of the major companies:
http://www.symantec.com/index.htm
http://www.mcafee.com/us/
Obviously there are many others as well. The important thing is to
get one and keep it up to date. Run a full scan of your system right
away.
4. After all of this you should be running much faster than before.
Lastly defragment the hard drive.
http://helpdesk.its.uiowa.edu/windows/instructions/defrag.htm
If you have trouble defragmenting start here:
http://forums.pcworld.co.nz/archive/index.php/t-27143.html
These steps will free up hard drive space, memory usage, and processor
usage. If you have any questions about any of these steps please let
me know.
source : google answer