A recent upgrade of the ZoneAlarm firewall software on a desktop PC knocked me off my Windows (2003) Domain. I was (suddenly) unable to connect to File and Printer shares on the server, where before they had worked perfectly.
After too much headache, it seems my server's IP somehow disappeared from ZoneAlarm's 'trusted' sites list when the update occurred...thus, ZoneAlarm was blocking File/Printer sharing with that machine. Adding the server's IP as a trusted site (and clicking that obscure little 'Apply' button) did the trick. If you use ZoneAlarm (or any other software firewall?) and suddenly lose access to network resources, you might consider checking for a 'trusted sites' or 'trusted zone' setting in the software...