WinXP login problem

My laptop (running Win XP pro) was part of a domain at work. I now work from home. Yesterday, I logged into the same domain (although it does not exist at my home). While playing around with it, I removed it from the domain and restarted the machine. Now, I am unable to login. Earlier it allowed me to select a domain at login but now it does not. I am unable to even login as an administrator – I am totally stuck. How should I go about fixing the problem ?