Got IIS or Apache running? Then you should have the 'ASPNET' user added to the list... just go to user account settings and set yours to default... ASPNET has guest access only, and is used to run some server side stuff. do NOT remove that account or you will get into trouble.
I don't know what you're talking about... :-/
Dunno which one appeared on your machine but my development PC (Windows 2000, Visual studio 6, 2003 and 2005) has the following list
[*]ACTUser, some sort of debugging account. Used by Visual Studio .NET
[*]Administrator, the standard admin account.
[*]ASPNET, ASP.NET user space for webservices. Don't remove
[*]Gast, guest access... for anyone not known on the machine. Usually hidden on XP
[*]IUSR_TWS2, default IIS account. Don't remove
[*]ivanzon, my day-to-day account
[*]IWAM_TWS2, account for cross-process IIS calls. Don't remove
[*]SQLDebugger, SQL server debug account, installed by SQL Server 2005
Can you tell me if the extra user is on that list?