Roger, the solution is simple if you installed from an install CD or DVD not the live CD.
Simply change the jumpers so that the Windows drive is primary and the Mandriva drive is secondary, (you did say that you could still boot Windows if it was the primary drive.
Now you will only be able to boot Windows so the next step is to start to install Mandriva again but in one of the first screens you should have the option to install or upgrade, chose upgrade, it won't find any thing to upgrade so it will run through very fast.
At the end you should be asked where to install the boot loader, install it to the MBR of the Windows drive.
Ken