This may be a problem with the Win7 machines not (in Microsoft terms) having a machine account on the server.....like in the NT days of old.
You may have to go to each WIn7 PC, remove it from the domain, then rejoin the domain to get the "machine account" accepted.
How do you leave a domain in Win7?
Right click on My Computer, select "Properties" In the window that pops up, go to the Computer name, Domain and Workgroup settings, and click on Change setting. In the System Properties box that opens up, click on Change Name (which you won't do unless you want to) and select Workgroup instead of Domain. You will need to give a workgroup name. The system may need rebooting after this.
Then do it all again, changing back to being a member of the Domain......and probably rebooting. Isn't Windows wonderful.....
Of course, the problem may lie elsewhere, but hopefully, this willy do it for you (laboriously).
Cheers
Ian