I just installed Microsoft Virtual PC on Windows Vista Ultimate x64 sp1. I then installed Windows XP sp3 into a new Virtual Machine. The Virtual XP machine could not access the network shares on the host. It could access shares on other PCs in my workgroup, and I verified that file and print sharing, public folder sharing, firewall settings, etc. on the host was all correctly configured. The workgroup name matched, it could even see the host, but kept giving me an error that I did not have permission to access this resource, and suggested I contact the administrator - not much help when you are the administrator.

In my case, the solution was to increase the IRPStackSize value in the registry:

1. Backup your Registry!
   
2. Run regedit.
3. Navigate to the following key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\LanmanServer\Parameters
4. In the right pane, double-click the IRPStackSize value.
   
   NOTE If the IRPStackSize value does not already exist, use the following procedure to create it:
   
   •  In the Parameters folder of the registry, right-click the right pane.
   • Point to New, and then click DWord Value.
   • Type IRPStackSize.
     
     IMPORTANT: Type "IRPStackSize" exactly as it is displayed because the value name is case-sensitive.
   
   
5. Change the Base to decimal.
6. In the Value Data box, type a value that is larger than the value that is listed. If you created the IRPStackSize value using the procedure described in step 4, the default value is 15. It is recommended that you increase the value by 3. Therefore, if the previous value was 11, type 14, and then click OK.
7. Close the Registry Editor and restart the computer.

If the problem persists after you complete the preceding procedure, try to increase the value of IRPStackSize even more. The maximum value is 50 (0x32 hex). I expect that this solution would also work for similar problems encountered when running a virtual machine in VMware.