A relative of mine had this problem a couple of times, so I thought I’d better document it.
When he’s starting MS Word, he gets this message:
"Buffer overrun detected!
A buffer overrun has been detected which has corrupted the program's internal state."
Word crashes afterwards.
The solution is simple. It appears the custom dictionary is corrupted. Renaming it solves the problem.
The file is located here:
C:\Document and Settings\<username>\Application Data\Microsoft\proof\
Rename custom.dic to custom.old, and the problem will be solved (hopefully).
You can also try to recover your custom dictionary by opening it in Notepad, and removing whatever garbage you find at the end of the file.