If htere is no virus info, then it looks like someone is playing with you.
Fisrt up, change all system admin passwords.
Secondly, look through the event viewer file s- the name change will be recorded there - that way you can establish when (day and time it occurred).
Given that soemone has probably cracked your admin passwords, they will do it again and again. You will need to start improving your security. Review who has sys admin password knowledge, but suspect other users as well. Turn on a lot more auditing.
Change the local NT sysadmin passwrods on workstations. Do users have admin rights - if so then it is possible its being doen from the user login - one person could simply be guessing user passwords and doing itfrom there.
You have a lot of frustrating time ahead of you - I've been through it and its not fun.
I suggest you look through all the security info at M$ sites and many others on the net. Ensure you have latest patches. etc