Actually, this is probably the easiest solution: just run bcedit from Windows.
EXAMPLE: c:\windows\system32\bcdedit /set increaseuserva 2500 c:\boot\BCD
1. "BCD" is the boot database, stored in XXX:\boot\BCD (where "XXX:" is your boot drive)
2. This is true regardless of whether you're booting via grub or BCD. The BCD file is there for any viable Vista/Server 2008/Win7 install.
3. "Bcedit" the BCD database, and you've set your configuration.
Voila. It should really be as easy as that!
'Hope that helps!