Changing user accounts locks up machine (apparently)

I have two user accounts on my iMac, mine (administrator) and my wife's. Occasionally I have to reboot the computer to clear out the cobwebs. I have tried closing all the apps for one user and then logging out (to shift to the other) and when I do the screen turns light gray (not the same textured screen at login but a solid gray) and appears to lock up as in nothing ever happens. Only resort is to depress and hold the power button.

I'm guessing it's a non-Apple app that is causing the ruckus but I don't have enough OS X experience to figure out which one.

Any ideas?

Thanks!

How are you changing accounts? Fast user switching or log-out/log-in (your post hints but just want to be sure)?

Normally fast-switching but when shutting down the computer I prefer to log out of at least one account first. It's the logging out that is the issue.