Did several of those get this problem simoultaneously ?
In that case, I suspect it may be due to a Windows update.
Just a friendly advice: Those computers probably have several years left, before the components (harddrive, motherboard, power supply) begins to die, so I would feel bad if not telling (no kidding).
If you installing Linux (I know that Fedora Linux X64, Cinnamon runs very well on an old Optiplex 780) on those computers, you may even not need to buy new ones.
Disclaimer: Yes I know it takes resources (work hours to install/maintain), and no - Microsoft won't (never ever) make Office/Outlook to work on Linux (and all those classic things that following migration to Linux). But there are great alternatives avaiable.