My space in the harddrive goes up from 3Gb to 11GB even though I didn't install anything to it, and I used it as usual. It's filling up by itself. How can I solve this problem?

Have you emptied the recycle bin lately?

Also, check the amount of space you allow the recycle bin and system restore and Internet Explorer to waste. To change the amount of space for the recycle bin, just right click on the trash can and press properties. To change the amount in System Restore, right click my computer, go to properties, then System restore. Click on the hard drives in the window and go to settings to enable or dissable system restore, and change the amount of space used. For internet explorer, just open internet explorer, click the Tools button on the toolbar, go to Internet Options, and in that box you will see a settings button for IE's temporary internet files. You can lower the size of that as well. Personally, I keep system restore down to 1GB, Internet Explorer I have at 50mb's, my DSL is very fast. I don't remember my settings for the trash can, but I keep it very low.