Download ad-aware, spyware doctor, and spybot from download.com, then install them and update them if need be. Run full system scans and delete whatever it finds. Then go to start>control panel>add/remove programs>and remove any programs you don't think you need or that you don't use. Also, go to start again>run>type msconfig>and then click the startup tab>uncheck any program you don't want running when you startup your computer or that you don't think you should be running. You should also do all the things TRD recommended and even do a error check of your hdd. Just go to start>open my computer>right click the hdd (should be c
and click properties>then go to tools and do the error check. It'll say you need restart your computer in order to run the check so do it. And make sure the settings for the check have it so that it will automatically fix errors, and scan for and attempt recovery of bad sectors. When you have both of those checked, restart and let it run. Doing all of these things will take a while, but should really help your computer.