This utility is mainly intended for freeing up disk space, but it can help demist a murky Windows installation. To run it, click: Start » All Programs » Accessories » System tools » Disk Cleanup.
It’s even easier in Vista and Windows 7, you just tap the Windows key and start typing disk cleanup, after a few keystrokes—in my case just 2—you’ll see something similar to this:
Press enter or click on the Disk Cleanup icon and you’ll be presented with the window below. Check the boxes for the files you wish to get rid of and click the Clean up system files button.
Note: if your Windows installation was an upgrade from a previous version you’ll have multi-GigaBytes of backup files from the old installation. You’ll only see these and be able to remove them if you run Disk cleanup as an Administrator. That’s easily done by right-clicking on Disk Cleanup and selecting “Run as administrator” from the resulting pop-up menu.