Cannot Install OS selector with DD11

I am having trouble with serial numbers with DD11 when I try to install OS Selector.

When I try to install the OS selector the installer asks me for a serial number, which I assume is my DD11 serial so I input it. It then correctly recognizes that this an upgrade from DD10, which I have previously used and registered etc.

The installer then asks me for the previous OS selector serial number, which I assume is the DD10 serial. I input this but it won't allow me any further in the installation.

I must have had the OS selector running previously as I have a dual boot system but have not booted into the second OS for a couple of years (it's XP and I now run Win7 generally).

If there is a way to force the boot into the other OS without the OS selector I suspect I will be able to rescue the old serial number (if it is saved there).

Any help would be appreciated!
Thanks!