Do your research and post questions on forums before you undertake any major house cleaning.
Stay away from your Registry(comp brains) until you have a comfortable grasp on what is entailed.
Start out with Spyware removers like Spybot, Adaware or MS spyware. You should notice an improvement.
Next, in your browser, go to the tools menu, find the options then locate the area where you can delete temp internet files and cookies. Delete these things. Check the settings for temp internet files. By default it's set at something like 1056mb. This is way too much. Every time you go to a website your computer searches this file to see if it has already stored the basic template for the site. Most times this takes up more time than actualy loading the site . Set it to 5 or 10 mb. This will speed up your browsing a bit.
Defrag your hard drive. Found in the start-programs-accessories-system tools. this may take awhile so sit back and wait. If you have never done this and your computer is 6mos. or older, expect to wait for 45min or more.
It also wouldn't hurt to run disk cleanup. Most of what this does is delete temp internet files and so-0n.
Organize your files and eliminate duplicates and stuff you just don't need.
Uninstall programs you don't use anymore. Then delete the program download to free up more space.
Certain programs use alot of ram (memory). When you run low on memory your computer will slow down or even freeze up. If you notice certain problems when you use certain programs this may be an indicator. Either uninstall then reinstall(to fix minor errors) or find a suitable replacement.
I hope this has helped you in some way.