Three DLLs that feature prominently (but not exclusively) in the rogues gallery are
CTL3D32.DLL
MSVCRT.DLL
MFC42.DLL
and Applications such as Flight simulator, AOL, Encarta provide their own and they may be ancient. This is a good reason not to install an old version of these programs or, if you must, try keeping some clean versions of these DLLs handy to reinstall after things go pear-shaped.
My MS Technet documentation will help diagnose some DLL faults and I'd be happy to look items up for PPRuNers if you publish the error message here or e-mail me. (You know the sort of thing, "Tcw50 caused an invalid page fault in module Mfc42.dll" etc when you try to start TurboCad).