If you are running 128mb or 256mb of Ram, it's pretty cheap and easy to upgrade to 512mb. Although this suggestion will actually cost money but we think only 450/- Rs will increase Your computer's speed much more than before. Remember to buy RAM of good quality. In Pakistan the trusted RAM is of Kingston.

Fancy sliding, fading and animated effects that windows uses by default can easily be turned off, and will make the reaction time of simple tasks like opening and moving windows, taskbars, etc... much faster.
From the Control Panel, click on the "System" icon. Click on the Advanced tab. Click the "Settings" button underneath "Performance". Uncheck the options related to animations, and other unneeded visual effects.

Indexing service gets info from files on the hard drive and creates a "searchable keyword index."
If you don't use the XP search feature often to look for documents, you can turn this feature off, and the difference you'll notice is a slight increase in the time it takes for your computer to find a file, but an overall increase in general speed for everything else is much better than it.
From My Computer > right-click on the C: Drive > select Properties.
Uncheck "Allow Indexing Service to index this disk for fast file searching." Apply changes to "C: sub folders and files," and click OK.

Windows stores a lot of temporary files that can be safely cleaned out once a month or so. This will speed up Your Windows XP and of course You will enjoy much better speedy computer then.
For this purpose You can use Free Software like CCleaner which You can easily download from software's section of Google URDU. So download it, install it and enjoy it Great Features, All Free and without any cost.

Its very important for a PC to Run in a proper way, You need a good antivirus program. There are many antivirus programs now a days in market some of them are free too but less in features. Anyway You can download AVG (Simple and Easy to USE Antivirus) and install it on Yours system. Its Freeware version is available at its site and We think its one of the Best Ant viruses Which are FREE.
You can download its Premium version and get its key from Google Urdu's Software's section also. In this way You can enjoy Registered Version of AVG.

Do not store media files like mp3, dat, ram, wmv etc Audio and Video Files in the Windows XP drive i.e. C-Drive. Use other hard disk drives for storing media files.