Follow these instructions to completely remove Apple software
Then use free Ccleaner to repeatly repair your registry until it's fixed.
download iTunes again from Apple.
If you have 64bit Vista or Windows 7, look for the seperate download of iTunes for that further down the page.
Once installed, use the Apple Software Update under your Start Menu to fully update all Apple components.
Your content will remain on the drive in the Music > iTunes Folder, nothing is deleted.
You should be making backups of your content regularly, either through iTunes or via copying the iTunesFolder in your Music folder to a external drive.
Windows is notorious for having "other" issues that make it unstable, we as volunteers helping others can't always resolve those issues.