iTunes problems after new password for Proxy-Server

Hey Folks,

I'm using iTunes (Windows 7) on my business computer at work. Our company is using a proxy server to connect to internet.

When I install iTunes for the first time on this PC I get the following Dialog to enter credentials.

After entering username and password - everything works pretty fine. Furthermore, I can check "save password" and I will never ask again to enter it. Very fine !!!

BUT:

In our compay there is guideline that engage employees to change password after 90 days.

After changing it, the connection to iTunes store isn't available any longer!!! iTunes saves this credentials somewhere on my computer and asked never again for new credentials.

Does anybody know where I can change the stored iTunes proxy password? Or how can I force the dialog above?

Every clue is worthwhile.

Kind regards

Torsten