ok.....hopefully i'm not being to dim and i'm presuming your machine is logging onto the wrong domain.
goto settings>control panel>network and stay on the configuration tab. The first option in the list should be "Client for Microsoft networks", click on the properties of this, and then change the name of the NT domain to the one you want to use. Click apply/Ok and bobs your uncle.....unless of course you dont have an uncle called bob.
Seanos