You may have a lot of stuff operating in the background, some of it you might not need. You'll see some displayed next to the clock. To stop any you want to stop, go to start, select run, then type in msconfig. from there select startup, and you will see all the stuff you have at startup. be aware of what you delete though as some may be systems stuff, and you can't always tell.
You may need to add more RAM. Have a look at his site:
www.crucial.com.uk/
They will automically investigate your PC and advise what you need and/or can have.