If you are a win7 user then this applies to you. It doesn't matter if you clean your system regularly, uninstall solidworks and start with a clean slate. Over time your computer will start to run out of memory regardless of all those things. This can happen quicker if you uninstall and reinstall programs often. There is a folder (C:\windows\winsxs) in the windows structure that keeps track of all the programs that were installed and nothing gets deleted from it. I have looked through many blogs that tell you how to reduce the file size safely. Microsoft doesn't recommend it, meaning they can't guarantee the application will work correctly. However many people have found them to work. There are several ways so just do a google search for "winsxs".
The best recommendation to make is to reinstall windows. Perhaps create an image of your drive and that way its quicker to get your machine back to its orignal state.