I just did that on my computer for me and my girlfriend. I'm set up as an administrator, and she is a 'user'. I installed the newest ICQ on my user account, and it worked fine. I shut it down and switched to her account. For some reason ICQ had to be installed fresh on her account as well - and it also set up perfectly.
Once registered, both users can be logged in at the same time to ICQ and can actually message each other. I was actually quite impressed by that.
