If you computer is starting to slow down and several years have passed since you purchased your PC, then it is time to rebuild.  Rebuilds usually include replacing the motherboard, PSU, memory and depending on whether your a gamer or not, video card.  Usually you can keep your case and power supply ($110 savings) and your monitor should be a LCD but, might  not really need replacing ($200 savings) and your keyboard and mouse should not need any upgrading either.  Of course you can also keep you hard drive too but, I find everyone needs more space and hard drives are getting less expensive every day.
Keeping this in mind, look at my current systems under the Hardware tab and you will see you can get a fast system using your current parts and a few new ones for as little as $210 for just PSU, MB, and RAM or $315 if you add a decent video card.
Call or send me an email and I will be glad to assist you in selecting the correct components for your new system.