Windows does it's own memory allocations, and InDesign is a 32-bit program so it can't see more than 3 GB anyway, so you have more than enough RAM (I also run Win 7 with 16GB of RAM and commonly keep Photoshop, Illustrator, Acrobat, a browser and email open, and soometime office applications, too, without a problem). Out of memory error are pretty rare and indicate, I think, some kind of document problem -- lots of complex vectors pasted in or many embedded images, or perhaps a bad font.
More information will certainly help. You might want to check the file size on any ID file that shows this problem. Anything over perhaps 20 mb is starting to sound large, and if you are looking at gigabytes there is a file problem of some sort.
Will post more for you tomorrow! Does ID create a crash file for this type of message that may help?
It produces wonderful crash reports.... on the Mac. Windows reports are pretty much useless.
Knowing what your users are doing, and how repeatable the problem is will help. In the meantime, take a look at Troubleshoot font problems | Windows on the off-chance it's a bad font. ID is more sensitive to font trouble than any other application on the planet, I think.
It would also be instructive to create a brand new user account for the person having trouble and see if things clear up. That points to a problem inthe user profile rather than something system wide, but if the problem is intermittent it may not be easy to tell.