Try single or double quotes or the DOS filename:
Code:
cd "/mnt/windows/Program Files"
--or--
cd '/mnt/windows/Program Files'
--or this may work too---
cd /mnt/windows/progra~1
And don't forget that it's case sensitive.
P.S. I'm not sure if linux sees both the vfat long and short filenames, so progra~1 may not work.