I hope that you backup your data, if so a clean reinstall would be best if you have an installer disk. Otherwise, a repair installation might save your data.
If you don't have installation media, then restore from the OEM partition, as I assume that you did not build this computer. Your manual will have a section on how to do that, but your data may be lost if you make the wrong selections, so be sure before choosing a path.
Perhaps you should consider using a freeware program like CCleaner in the future though, it is good for cleaning up computers but keeping important stuff.