Brought a win2000 machine home from work that has my software installed on it so I can make the changes to the 3D-Vert PCB.
hooked it up at home
logged in with with username from work
changed workgroup to point to my home network
now I can't log into this machine at all.
Do you have the admin account info? Try logging in with that. You didn't need to change the workgroup at all. Even if you have a domain at work you just needed to login to the PC locally.
If he did use a domain at work, normally it would ask for a username/pass to change the computer workgroup/domain settings. But if there was no domain/login server/roaming profile at work, then just changing the workgroup shouldn't have any impact on logins.