As it said on the page you used to submit a thread, please post this sort of inquiry in the "Mapping Help" thread. We are a friendly bunch if you respect our guidelines.
A Few Things That Might Help
- Disable visual styles for the program.
- Uncheck the "use win32 file dialog" in the options.
- Manually add the .map extension to the file name when you save.
If I do the above GtkRadiant works for me under Vista, hopefully you can get yours working too.