Interesting.... I've never come across this problem. Also interesting is the first link Lonk linked to, talks about clicking on "Manage Default Printers" button. I don't have that button at all on my desktop. Another site suggest disabling the change with reg hack...

writevalue("hkcu\Printers\Defaults","Disabled",1,"REG_DWORD")

http://www.normanbauer.com/2011/03/11/how-to-disable-windows-7s-default-printer-management/