GRUB doesnt affect this at all. The reason being is simple. GRUB and Linux doesnt overwrite the System Active partition for Windows. As where installing another version of Windows can. Since the install of XP was done after the upgrade to Win7, that isnt the case. But if the machine had XP first installed than Win7, then it would have caused an issue. So for you the addition of Linux meant nothing. But adding in another Windows install does mess with the MBR and BCD.