I repair a lot of consumer computers and it is challenging because they can all be very different (I enjoy that
Does your user account require a password that the tech did not know?
If so, perhaps he chose to create a new account rather than try to login as administrator or ask you for the password. He may have needed to reactivate XP after adding a new motherboard or as you have suggested, change or add drivers.
Not the choice that I would make, I like to call and talk to the client about everything as I do the job. Many techs prefer to just sit down and do the job in one shot so they will do things like create their own user account.
I doubt that anything malicious occurred but if you are concerned, call and speak to them about it and let us know what they say and we can tell you whether or not it makes sense.