Here is something that should be bomb proof.
Put Grub into a Primary partition. Say if your Ubuntu is in hda3 (only hda1 to hda4 are primaries and one of them can be converted to the extended partition). You can do it while in Ubuntu issuing a root terminal command
Thereafter Grub can be activated by booting into hda3 using Windows MBR.
If your Ubuntu has been installed in logical partition you can still installed Grub in a data-only primary partition. Just follow Section C of my 2nd link in the signature.
You can then restore Windows MBR overwriting the current Grub so that the PC is booted to Windows on a reboot.
Any time you want to switch back to Ubuntu you can click start/my computer/storage/manage/disk management and ask Windows to make the Grub partition hda3 active (it will not be called hda3 but you should be able to identify it with a label). On a reboot the Windows MBR will boot up Grub.
After you have enough of Ubuntu and want Windows back then just use Grub to make Windows partition active, assuming it is in hda1 and so known to Grub as (hd0,0) because it counts from 0, by the command
You can switch between the two remotely.
After explaining this bomb proof method, which I have used myself, I could not understand why can't you would just add couple of "0" to the timeout command to give me loads of time to click between Ubuntu and Windows, like
Anyway I beieve the above two methods should satify your need.