It's fairly simple nowadays. I presume you are using either Vista or 7 because of the Aero design, so the only question is if you have an installation CD or not? First of all you need to backup any files you may need later on. Then just pop in the CD and I think there is an option to reinstall or install in there. It's important that you don't try to repair, I would just wipe the hard drive that has Windows on it (maybe write down the hard drive letter and how large it is, so you don't wipe the wrong one if you have multiple hard drives). Format it to NTFS. There is no point me giving you an exact description of every step, because honestly I'm not sure what the entire sequence is and what everything is called in the English version, so here is a link from Google on how to reinstall Windows 7:
http://pcsupport.about.com/od/operat...all-part-1.htm