http://forums.arcgis.com/threads/41396-Managing-ArcGIS-Server-through-ArcCatalog
Here's what I'd do to test:
A. Create a local user account on your server machine. Name it whatever you like.
B. Create a local account with the same user name and password as the local account on your PC. Add it to the AGSADMINS group.
C. Launch ArcMap from a command line using the RUNAS command (you can get usage help by entering RUNAS /?), and open it as the local user you just created.