Here's what I would do. Install more ram. You should have at least 2 gigs. To check what your computer has, click "start" then go to "run" and type in "dxdiag" click enter. click yes if prompted to. It will probably show you your ram in megs. So... You want at least 2 gigs and 1 gig equals 1024 megs. If you computer is older, it will have somewhere between 64 - 512 megabites of ram i think. Anyway, the dxdiag report should also tell you what kind of ram you have. Then you can go to best buy or just find it on the internet and buy it. It will need to by physically installed into your computer, of course, because it's like a big computer chip. To see how to install it, once you know exactly what you need, look on youtube. There are some good tutorials that can walk you through the process.
Also, here's something that is free that you should try. click "start", then "programs" then "accessories" then system tools" and finally "disk defragmenter" or defrag or whatever it says. Defrag your hard drive. What this does is it finds all the bits and pieces of storage in you hard drive that are isolated and puts them all into one big chunk. It's kind of like a cluster of bubbles. Assuming they can't pop, well then you can't really store much in them, but if you put them all together into one giant bubble, then you have all this room to store things and move about!
Also, click "start", then "control panel" then "internet settings" and delete your cookies, internet settings, all that. Sometimes spyware and that sort of junk can be hidden in there.
Finally, go to download.com and download the AVG free program. Install it and do a full computer scan. Viruses can slow down you computer immensely! You could have a bunch of trojans and spyware, and AVG free will detect and delete it all. And I recommend this program because it works very well and it's free! Plus, download.com is a VERY safe worry free site.
Well, that's about all I know... Good Luck!