How to change iTunes Proxy User name / password?

I am using iTunes in a company environment with a proxy. Itunes kept asking for proxy username and password so I gave it to it. But we are required to change passwords. Now itunes just crashes. I have tested iTunes outside of the office and there is no problems with it so it is the proxy settings that cause the problems.

The proxy is fine, no other programs have a problem and both IE and Firefox work with no problems.

I have uninstalled itunes and tried to delete all files. This did not help. This includes deleting all files in "AppData\Roaming\Apple Computer\Preferences"

So where does iTunes actually save the user name and password and how can I change it?