I wouldn`t buy a new one if all you`re worried about is speed, do as someone else suggested, format and reinstall, you will be amazed at the difference.
I too reinstall about every six months, I back up regularly, so it`s no problem.
If you decide to do this, make a driver disc first (unless one came with your pc)
To do this, download drivermax, export your drivers and copy to cd.
http://www.innovative-sol.com/drivermax/
You could probably also upgrade your ram, very cheap and easy to do. Scan with crucial, which will tell you exactly what you need. Write it down and buy from somewhere cheaper, I`ve had some very cheap ram from ebay.
http://www.crucial.com/uk/systemscanner/index.aspx?click=true