This is a known issue with Windows XP, Windows Server 2000 and Windows Server 2003, but a number of users have reported that the solution explained here also fixes Windows 7.
The installed value of the IRPStackSize parameter is often too small to allow additional network clients to connect to the Windows file sharing service.
Microsoft have published Knowledge Base article KB285089 which describes how to increase the value of the setting to fix the problem.
Here are the instructions for making the change, but see the Microsoft article if you need more help.
(The name is case-sensitive)