I'm suprised this doesn't work:
Go to Control Panel --> "Administrative Tools" --> "Local Security Policy".
In the Security Settings branch, select Local Policies --> User Rights Assignment.
On the right hand side, double click the setting for "Access this computer from the Network". Check that "everyone" is on that list! If so, great, it should work, but apparently doesn't in which case it's beyond my knowledge. If not, "add user or group" and go for it.
I've said everything I can imagine. Think you might need to email/call Microsoft.
