Word definition includes period?

Hi,

While using vim 7.4 on cygwin, I noticed that word characters include a period, for example "cw" with cursor on the f of file.html attempts to change the entire file.html rather than just file. I have LANG set to C and "se encoding" shows latin1, and there's not much in vimrc (identical to the one on Linux where this works as expected). What may be the problem here? Thanks,

saurabh