That account really should not show up on the Windows 7 screen either. It sounds like a system account that the program uses, not a real user account. And if that program does actually create a user account for itself, that's pretty bad.
What type of account is this postgres? Does it show up in the User management?
The postgres account is a normal user account without admin privileges. It shows up in User Management on XP, through the manage UI, but it does not show up in the XP Control panel.
On windows 7 you can login to it, which is normal. But on Windows XP there seems to be no way to log into it.
In general the postgres account is only ever used when starting the postgres service.
You can log into postgres in XP. In control panel, user accounts, select "change the way users log on and off". Turn off "use the welcome screen". Now you will have to type in your user name and password to log in. I'm sure win 7 has the same option.
find the user name and edit the reg_Dword
for hidden set it to 254 I believe, and to unhide it set it to 0.
Windows 7 is the same Idea.
but remember, windows 7 disabled the CTRL+ALT+DEL functionality, so unless you remote desktop into that user from another station that account isn't accessable at all unless you unhide it.