A common complaint about computers is that it's running slower than usual or just is plain bogged down. Though this can be a sign of spyware or antivirus infection it can also be a sign that your hard drive is filling up with clutter. Mainly of windows temp files, internet files, unused programs and programs running in background etc.. Luckily Windows does include built in programs to help combat this. The one used for clean up is called(surprisingly) "Disk Cleanup". Which will cleanup temp files, unused programs, internet temp file. It can be found by clicking Start,then All Programs, go to Accessories, then System Tools, and then Disk Cleanup. Windows will estimate how much space you'll save. It sometimes takes several minutes ,so it's a good time to fetch another coffee or see if your kids are terrorizing your pets and so on. Other options included are uninstalling programs you don't use anymore. One other option is to remove all restore points except your last(good one). To do this click on the more options tab and then select Clean up in the System Restore dialog box. Another option is to use a third party clean up program like CCleaner which goes a little more in depth. Did I mention it's freeware? Good luck.
For more info you can check out the Microsoft support site and/or Microsoft Technet.