OS selector - unable to activate

I have two OS, both XP SP3, one on each of two internal hard drives. Following a disk defrag on both disks on next boot I received MBR error 3, Windows console repair 'fix mbr' cured that , my DD 10 shows both OS, but I can only boot, automatically, to the OS on the D drive. When I try to select C drive from the DD OS selector window It just tells me to activate OS selector, when I try to activate the OS selector the error message informs me 'unable to activate OS Selector'. Can someone help me please? Thank you