When the GRUB boot screen comes up, the first line is highlighted, and
that is the OS which will boot after X seconds, if you don't do anything.
Count from there (starting with zero) until the line with the Windows
entry.
Then edit in Linux the file: /boot/grub/menu.lst Find the line:
default 0 , and change the 0 to number of the line you want to have
highlighted. Save the file, and reboot.