In 2005 it asks you what you would like to run the agent under. I choose the service account. It warns you that this isn't the best security practice, but it works.
If you want to use an account other than the service account, you need to make double sure that that user has rights to the directory you are creating the snapshot in. I find that if I'm having a problem, that rights on the directory that holds the snapshot are the cause 95% of the time.
Good luck.