Network Issue

I have win 2K server & XP Pro clients. every 1/2 an HR. or so when I try to access my Server from any PC or say I try to Print on any shared Printer on the network its asking me to enter username & password to access it. WHY???? If need to print 1st I have to provide username and password of the server then only i can print or use any file or folder on the server or on users PC. WHY?????

My network has static IPs

any help will be highly appriciated.

Thanks & Best Regards

Huzaifa