When working with clients to install BizTalk Server I find it interesting that not many of the developers understand what is meant by service account - insofar as creating the account that BTS will run as.
The service account is a user account on a stand-alone computer or on a domain. When you create an account for BTS to run as, you are creating a service account.
Often I use the ID BtsService whether stand-alone or on a domain. This account is a member of the Users or Domain Users group by default, and should have NO OTHER permissions assigned to it (on several engagements I have found this account to be a member of the Administrator's Group - and this is not necessary, and could allow security breaches).