Filesharing problem from ONE client on ONE PC

40 clients can access fileshares on our x-serve from their desktop mac or from any office/clinic PC.

On PC log in with employee's username and password

use RUN command eg RUN \\123.456.78.90

a Windows screen asking for username and password appears, enter their MACINTOSH Username and Password and they see the shares they are entitled to.

One user, can access shares from ANY Mac she logs into and from any of 100's of pcs across the enterprise using the above method
BUT not from her own PC .

The RUN command comes back with << not authorised to use this...contact System Administrator...>>

If anyone else (Me) logs onto her windows PC and uses RUN, they are presented with request for Mac Username/Password - enter this and we're in...indeed if at this stage we ask HER to enter her Mac password, access is granted.

So:

she can access from all macs - so Mac Server permissions, Username/Password are OK

she can access from almost all PCs - so Mac Server permissions, Username/Password are OK

she can access from almost all PCs - so her PC account is not at fault?

Others can access from her PC - so it's not her PC Box at fault?

The enterprise IT are happy to blame Mac and pass the issue back to me, but I can't see that the mac side of things is at fault...

Anyone?