|-Cut out unnecessary background programs by going to run > msconfig > startup and unchecking programs you don't need to be running. If you're not sure of what some programs are, you can try googling the name, but Windows won't let you uncheck anything it needs anyway.|
-Use a program like Spybot to remove any spyware that may be sucking up your system resources without your knowledge. I like to use a second program like Ad-aware to double-check. Both programs have free versions available, just google them.
-Run a virus scan to make sure no viruses have infected your system either. Avast offers a free version that does a nice job. AVG is another free program.
-Defragment your computer so it's not taking the long route to access your files. The defragmentation program is in your system tools folder under accessories.
-I personally would get rid of crap you don't need to clear up space on your hard drive. In your system tools folder, there's something called Disk Cleanup that will help you get rid of wasted storage space and compress your files. In your control panel under Add or Remove Programs you can uninstall stuff you don't use too.
I have had my computer bogged down and running slow, and after I did all the above steps, it began running the way it should again. If it's still running very slow, perhaps you need more RAM, which determines how much work your PC can do at one time. The more RAM, the more tasks it can handle. Computers these days should have 2 gigabytes of RAM, I think. I have 4 because I like to multitask. You can find out how much RAM you have my going Start > and then right-clicking My Computer > properties. Hope that helped.