Take a look at this wikino MBR found
Can't say for sure if it is something along these lines but a change to UUID may do the trick (explained in the wiki)
Take a look at this wikino MBR found
You're an excellent guesser. During installation, Windows will search to see if there is an already existing Windows OS in any computer they are being installed in. If they find a pre-existing Windows installation in a primary partition, they are programmed to set up a 'Microsoft Default Dual Boot', and install the new operating system's boot loader files in the older operating system.My guess (I am not an expert here):
7 installed its boot information onto the XP partition (as it was the "first") and when I installed linux, I killed that information (MBR?) and now 7 cannot boot anymore.