I have a few Windows machines (and virtual machines) that connect to an Apple Xserve. I had some issues with certain folders not being displayed recently and it came down to a folder naming issue. If there was a space as the last character, the PC could not see or browse to that folder, even if it were keyed in manually with the space. Once the bad folder name was corrected, Windows access worked fine again.

References:

http://support.microsoft.com/kb/320081