If you have an AMD Athalon then it is probably a socket A which is easy to upgrade. For example you could buy a faster athalon or a socket A sempron/duron. These processors are not expensive at all, but it's worth checking out compatibility with your board on the manufactorers w/site before buying (if your in the UK then check out
http://www.ebuyer.co.uk for some really good amd deals!
)