Wow...that entire login process was totally unnecessary and glitchy. Amazing.
Ummm I just had the same thing happen as above and it took about 10 different search attempts on bing (really google! lol) to find this single yell for help.
This is the second time this has occurred to me. I think I have it figured out - love the System Restore suggestion - why not just tell him to reinstall windows??
Like I said after several useless searches I found this and when i was trying to read I realized that there was a floating 'OPEN' or some other 'PROMPT' button floating on my screen on top of my window - it was white and not easy to see and well it appears to have been from Microsoft Security Essentials - I had 3 programs running and every attempt to access whatever was not on top yielded the flashing orange no you can't see it '!!@#!#@' unless you minimize etc.
Once I clicked the 'OPEN' button - then Microsoft Security Essentials popped up on top and wanted me to do a scan. It did not show up as a running program on the bar as it is in the try so.. no task button to select.
I believe it popped up and I was doing something (gosh why would a program just pop up when I am typing and steal the focus?) as I say I was typing so it almost immediately was pushed to the background - except for the little white button floating on my white word doc - almost invisible. Once this state occurred I was unable to switch tasks till I hit that button. When the user minimizes the task switch function works but there is no way to see that MSE is the issue.
System Restore... yeah that fixes everyting