You can set it to any otherm, "real" user account. But you will have to be carefull.
1) Apache still needs to run properly. (duh!)
2) Beware of security implications; making the user a domain admin will make sure it always works, but if someone does hijack your server, they have access to your entire server farm.
I have never used Apache on Windows, or done much with windows security, so you will probably need to consult the manuals.
As a test, change it to be a highly privilidged user, just as a proof of concept, but you'll want the user to have the minimum amount of access but still able to run.
Good luck!