Dialog box opens behind current window instead of in front

Unpredictably, when the PC (Windows XP Pro, AMD Duron 700 MHz, 192 MB RAM) has been in use for a while and I'm working in an open window (could be IE6, Outlook Express, Word 2002, or any application) a message box or dialog box such as Open or Save will open in back of rather than in front of the currently open window. I don't mean a pop-up window, but a legitimate dialog box opened by the current app. This situation makes it impossible for me to work in, move, or close the current window, because it has lost focus to the dialog box hiding behind it. Since I can't see the dialog box, I don't know what options are available or selected, and therefore I can't know what to do with it. The only thing I can do at this point is to press the ESC key to close the dialog box and regain control of the window, resize it to about 1/4 of the screen, and do the same operation again. Then I can see the dialog box, drag it into full view, and complete the necessary operation. Rebooting solves the problem for a while, but then it starts again. No error messages ever appear.
If I were using Windows 98 or ME I would guess that I was running out of "handles" or resources, but this is Windows XP, which is not supposed to suffer from that kind of problem.
Can anybody help figure out what is causing this and what can be done to prevent it? Thanks!

Frank D