Running scripts with multiple users logged in

[Snickerdoo]

I'm running a script on a WinXP machine that has multiple users logged in. I've discovered that when I switch users, the script running in a background session mostly continues to work, but certain window-related functions fail (e.g. PixelGetColor, which is not entirely surprising). I tried searching the AutoIt help and this forum for information about running scripts in the background but didn't find much, and am left with the following questions:

1. Is there a way that a script can detect whether it is running in a background session, such that I can have it avoid doing the things that fail but continue to do other stuff?

2. Is there a way my script can tell XP to switch users/log in a user/log off a user?

BTW I am using version 3.2.4.9, WinXP SP2 with the latest patches installed.

Thanks!