Boot Loader Option

I have a NanoPC that has a 32GB SSD drive installed. The SSD currently has Windows Installed. In addition I have an SD card that has a Linux installed on it (PBX In A Flash). If the SD card is installed I can set as the 1st boot option in the bios. If I remove the SD it defaults to the SSD inside. If I reinsert the SD card, the BIOS still defaults to the SSD drive. I would like a custom boot loader setup on the SSD (using GRUB, or whatever will work). When the PC is started, the bootloader should check for the presence of the SD card. If the SD card is found the boot loader should boot from it. If the SD card is NOT present, then the bootloader should boot windows XP from the SSD internally.