Everything that Roffey said, plus go online, do a search and get "Eusing Free Registry Cleaner". It's free and when it runs it will first make a backup copy of your registry, (so that if it makes a mistake you can restore your registry back to it's original configuration), then it will search your registry for all files, folders, start menu items, etc. and anything else that is no longer on your computer but is still in your registry. (Basically, when you start your comp, Windows looks at your registry and tries to load whatever is there. But if they are dead end files it takes time for windows to try and load them and then go on to the next file, etc.) By cleaning up your registry you speed up that process. This is my go-to registry cleaner. It's fast, effective, safe, and free. What more do you want in a program? Oh, and the user interface is sweet...very intuitive, very user friendly.
Oh, also, if you don't want to take up disk space for your antivirus checker/cleaner, you can go to Trend Micro and get Housecall
http://housecall.trendmicro.com/. This is an online antivirus program from one of the leaders in business antivirus programs. It's free and when you run the program it will install a small cookie onto your computer to enable it to launch when you click on it... but it keeps all the virus definations and the main program on their computer where they are safe from hackers. It will scan your comp in several different ways, depending on what you select, kill or quarentine any virii it finds, and it runs in the background so you can continue doing whatever it is you are doing on your computer at the time. It only runs when you manually launch it, so it's not taking up memory or bandwidth.