Probably because I was used to the structure of the unix core on Mac OS X, I just got used to having 'root' underlying everything.
When I moved to XP, I kept in the habit of running an admin account ; only doing system specifics like backup / virus scans / program installation etc.
For me and my partner I always made standard accounts for everyday use.
On XP I never had problems with a virus or trojan - honest.
With 7, I have done the same. But as there is now an administrator embeded into the system I will enable that, and delete my original 'admin' account.
I can't actually understand why people are so fearfull of running in standard user mode, surely it's safer, especially if you're a download or web monster