Just did a spell check on a document in 10.0.1. Dialog freezes for 30 seconds then a full crash. Front end and kernel shut down by windows. On Win 7 x64 and Mathematica 10.0.1. Save your work first!
I have the same problem, Windows 7, Mathematica crashes when I invoke the spell checker.
I am not seeing this issue on OSX.
I did indeed have text cells as introductory material to sections.
This is what I've been talking about for some time. WRI Support acknowledges the bug:
"With text cell the crash is reproducible and has been reported to the development team."
I think this is only a Windows issue. Also the use of Ctrl+K in a TEXT cell also causes a crash.
The fact that this got through extensive beta testing is probably an indication that very few users make much use of Text cells!