Excel 97 Close Button

Recently our users have had their close buttons become grayed out. (The one in the uppermost right hand corner) The only way to exit Excel is to go to the menu bar|File|exit.

SUPPORTING FACTS:
SR1 & SR2 have already been applied to Office 97 sometime ago. Win95, 98 and NT all are exhibiting this dissappearance. Reinstalling over Office will not cure the problem. You must completely uninstall, registry erase and then install office to clear.

Any suggestions of where to start looking?