if i reinstall itunes can i keep my old library?

Uninstalling & reinstalling iTunes should not affect your library, but you should backup anyway. 😉

tt2

From : http://support.apple.com/kb/ht1923 comes the following information:

Notes:

• iTunes Store purchases or songs imported from CDs are saved in your My Music folder by default and are not deleted by removing iTunes. While it is highly unlikely that you will lose any contents of your iTunes Library when following these steps, it is always a good idea to ensure that your iTunes library is backed up. If you're unsure how to backup, see iTunes: Back up your iTunes library by copying to an external hard drive.
• These steps may take a significant amount of time to complete, depending on your system.

Hope this helps 🙂

Stijn

As turringtest noted, always make a backup, especially before you reinstall anything.

If you format the drive and reinstall Windows, everything will be deleted. Even if you do not format, if the drive is messed up enough, things may get deleted.