This last week I had an issue, actually at home but is useful here, with my windows 7 machine beign able to map a drive to my 2003 server (Kept saying invalid username or password). After all was said and done updating windows was the fix. apparently there was an update to the network Security on windows Vista and higher, and Windows 2003 Server and higher that they must both be on the same version or it is unable to communicate properly. So if you suddenly get a client with Windows Vista or higher, and/Or Windows 2003 Server or higher that can’t seem to connect to the server ((or another windows 7 box I believe)) try updating both of them for all windows udpates it jsut might solve your network problem.
This does not effect Windows XP for some reason. So Sure sign it’s updates is a windows XP machine connects fine but a windows 7 doesn’t