I found that a number of Windows files copied across to an external HFS+ hard disk from a Windows PC using MacDrive showed as Unix executables.
It turned out that they files had Execute permissions set for all users.
You can change the "Execute" setting using chmod in the terminal, but a simpler solution is to install FileUtilsCM, a free context menu control available from
http://www.abracode.com/free/cmworkshop/file_utils.html
It offers a variety of high level controls, including remove file type and creator; remove resource fork; clear "execute" Unix attribute.
I found this fixed the problem for my files. The "clear execute" control fixed the file type, and the "remove type & creator" command converted the type back to Plain text from Document.
I don't know if it will work for your problem, but could be worth a try. Works in Leopard 10.5.1 for me.
Slightly more control is available from Super Get Info from Barebones
http://www.barebones.com/products/super/index.shtml
That's not free, however, but it does allow you to set the file type and creator.
DN