Windows 7 User Account Control (UAC)
When logging on to Windows 7 as a user that is a member of the Administrators group your user token is split into two tokens; a full administrator token and a standard user token.
The latter token is used to start the desktop (explorer.exe) and as all applications inherit their access control from the desktop they all run as standard user.
That is why it is important, on occassion, to select Run As Administrator when executing a program.
This can be particularly important when installing software - as I found out recently when installing the SAP Front End.
Lesson learned.
UAC details can be found in this Microsoft article.