A couple of other things to try...
Remove unnecessary software: if you've had the computer for a while, you'll be surprised the amount of stuff loaded you probably don't use.
Stop unnecessary programs from loading at startup: except for your anti-virus program, very few items actually need to load at startup. Look at your system tray, if you've got more than a handfull of icons, then you can probably dramatically improve your system perfomance. Click Start| Run type 'msconfig'. Tinker with this and see what works best for you.
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