Oops! Changed from domain to workgroup and now can't logon.

I have a laptop from work that is a member of a domain (XP Pro). I was
trying to share files with my home network (XP Home). The network setup
wizard wouldn't work on the laptop because it was a member of a domain. In
the Computer Name tab of the System dialog box, there is an option to change
the domain and switch to a workgroup. I clicked on the workgroup option
which disabled (greyed out) the domain name textbox. When I clicked the
domain option button it restored. Seemed reversible enough.
So I clicked the workgroup button and filled in the name of my home
workgroup. When I clicked OK, I was prompted to restart, which I did.
The laptop has secure sign in enabled, where you must press ctrl-alt-delete
to start. Then enter password and logon.
I now get a message:
"The system could not log you on. Make sure your user name and domain are
correct, Then type your password again. Letters in passwords must be typed
using the correct case."
There is an options button, but it only gives the option of shutting down.
The domain textbox is no longer displayed.
I did not enter a new username or password.
I have tried the system restore at bootup, but it is password protected and
does not recognise my password.
I dread having to go to the sysadmin Monday and admit my indescretion.
Any ideas?