Can't Open Safari and Word at Same Time

Urged my daughter-in-law to replace her old PC with a new iMac. she did and loves it. But now ther is this glitch that she has asked me for help with. I am stuck — thinking there was a simple selection item somewhere. But if there is, I can't find it. any ideas? Basically, the problem is this: when she trys to rum Safari and Word at the same time, one window disappears when the other application is selected in the dock. Then the other application will appear when selected but the one in the window disappears — nothing crashes. Here's how she describes it:

I could see both windows when I had iphoto and itunes open, but it wouldn't let me have itunes and word open simultaneously, or itunes and safari open simultaneously, or word and safari open simultaneously (as we already knew). I could have iCal open with the other programs at the same time (I just wanted to try another program in the dock to see what happened). I dunno... still it doesn't seem like a faulty operating system.... just a quirky way of organizing opened windows.... I wonder if it has to do anything with the function of using the command + tab function to scroll through open programs. I vaguely remember setting that up, or finding that function somewhere when I first was getting used to this computer and maybe that has something to do with it.

Any ideas?