I don't use PostgreSQL on Windows, but it appears the Postgres creates a new user in a way that you don't see this account from Control Panel->Users, but you can find it if you look into your "%SystemDrive%\Documents and Settings\" directory.
(Typically C:\Documents and Settings)
C:\Documents and Settings> dir
Volume in drive C is System
Volume Serial Number is 50X2-9512
Directory of C:\Documents and Settings
04/15/2007 10:43 AM <DIR> .
04/15/2007 10:43 AM <DIR> ..
03/29/2007 09:44 PM <DIR> Administrator
03/20/2005 10:00 AM <DIR> All Users
04/05/2007 03:08 AM <DIR> Michael
04/15/2007 10:43 AM <DIR> postgres
0 File(s) 0 bytes
7 Dir(s) 45,362,515,968 bytes free
And if you opted for creating a detailed installation log when installing Postgres (seems like a good idea anyway) you will find the username there as well.
P.S. And thank you for searching and appending to the relevant thread, instead of just creating a new post!!