SQL 2005 creates a number of local groups during the install process.  BOL has the full details of this.  They hold the service accounts used to run the various SQL services.

I have looked at deleting these groups but have decided against it.  If you look in the SQL portion of the registry, you will references to the SIDs of some groups, the names of others, and prefixes for the rest. 

There is no Microsoft or newsgroup documentation on what impact there will be on SQL if the groups are deleted.  If anything does break, Microsoft may well ask you to reproduce the problem using a standard environment (with the groups) before they can properly support you.  We have a regulatory requirement to use vendor-supported software, so for us the groups have to stay.