Right-ckick on [My Computer] and click on the [Performance] tab.
Look at the system performance rating.
This should be below 80%.
The higher it is, the slower it will function in completing commands.
Use the [Add/Remove Programs] as was sugested to remove applications that you no longer wish to use. Make sure that you do not remove any Service Packs or you will get a blue-screen of death.
This will free-up space but there may still be applications or other running in the background.
[ctrl][alt][delete] will show you what is running in the background.
[start][run][msconfig][start up tab] will show you what is loaded during the start-up process. You can turn things on and off on this tab but you have to know what you are doing or you will end up with a large paperweight.
It would also be a good idea to instal and run Adaware to make sure nothing malicious is running in the background.
Adaware Download
.