Boot from a Windows 7 install disc, and use the net user commands to make a new administrative user from the command prompt. Does that work?
It's strange your colleague deleted an account he was logged into. That shouldn't be possible, but if it was the only one, I don't think there is another way to do it.
A good rule, by the way, for a business laptop: if anything happens with your laptop that we can't fix in half an hour, we put the default image on in. Don't save anything on it. Only save files on the file shares we offer.
One of my colleagues accidentally deleted his Windows 7 account, and unfortunately it was the only account with administrator rights enabled - the default administrator exists, but is disabled.
Is there any way to enable the default administrator, by using another not-administrator account?