1. Use Control Panel/Add or Delete Programs to delete any programs you do not need. You probably have a lot of these. If you're running an older version of certain BIG-NAME anti-virus programs (hint: One sounds like "Horton" and another is something like "McAfoo", older versions of which are NASTY resource hogs), delete these and replace them with a brand new version of them or something completely different like AVG Free.
1a. Get a utility that allows you to modify your startup programs and deactivate anything you don't need (for example, a lot of programs like RealPlayer, WinAmp and WinZip run "agents" that you can usually do without, and a whole lot more totally useless cruft is invoked on startup on nearly every PC on earth). Take care, though, that you don't terminate something you might actually need.
2. Run a registry cleaner like Tweaknow Registry Cleaner, which is freeware. Delete everything it finds and marks "Safe to Delete." Then reboot and run a registry optimizer like the freeware NTREGOPT. Reboot again.
3. Run a good disk cleanup utility like Webroot's Windows Washer. As the first step, run a cleanup on your drive. Fair warning, though: Make sure you haven't "overconfigured" it because it might delete files you'd rather have kept. It's particularly nasty with older versions of Forte's newsreader "Agent," where it deleted my entire subscribed list.
4. Run a couple of adware/spyware FULL scans. I run both Spybot Search & Destroy and Ad-Aware, both freeware. Each finds different things and misses different things but both together do a pretty good job.
5. If you haven't already done so as part of Step #1 above, run an anti-virus scan. In this case, though, you should only have one of these installed and only run one. I have never had an infection get past AVG Free.
6. Run sfc /scannow from file/run. This will repair corrupted system files, and if your computer has ever crashed (whose hasn't?) you certainly have some of these. They can really slow your computer down.
7. After reboot, run Windows Update. Sometimes sfc will replace a patched file with its original version, and you will need to repatch it.
8. Invoke Webroot's Windows Washer again. Run another disk cleanup, which should only take a few seconds to complete. Then go to Wash Utilities and run a "Wash Free Space." A single pass is enough, but even this takes quite a while.
9. As the last step, run a disk defragmentation pass. The one supplied as part of Windows is marginally OK, but MUCH better and more effective defragmentation is achieved by programs like my favorite, Raxco PerfectDisk. This can be obtained for free for a 30-day trial, but after you see how much it improves your computer's performance you'll probably fork over the $40 bucks. Run a scan, which takes only a couple of minutes, and then take the steps the program suggests, which will probably include a "Smart Scan" of your entire drive and an "Offline (System Files Only)" pass. Reboot after each step.
After you take these steps, your computer's performance should be noticeably, perhaps even shockingly, improved. The whole procedure will take the greater part of a whole day, but several of the steps (the full anti-virus scan, the free space wash, and the disk defragmentation, will take enough time themselves, and run just fine completely unattended, that you'll be able to do other things, like grocery shopping and whatnot, while they run.
I hope this helps.