If you are having problems with Windows ( 7 , Vista , XP ) and don’t want to reinstall ( which should be the preferred solution ) there is a solution from Microsoft
Microsoft have the Microsoft Fix It Center Online
After installation it will scan your computer to diagnose and repair problems then it will attempt to fix the issues it has found.
This has proved especially effective for me with Windows Update issues on clients computers.
This is well worth a look for some issues but if you are heavily infected with malware/virus then a reinstall of the operating system is your best choice.