if you dont find another solution, run the live dvd and begin the install process, in the begining of the install process you will be given a chance to reinstall the bootloader only. read and watch carefully and should be able to reinstall the bootloader and boot into sabayon again.
edit*** you can also try to install easyBCD in windows which will add linux to the windows boot menu, but depending on what was destroyed this may or may not work. im guessing the menu is still there but written to the partition, not mbr, if so easyBCD will work.
edit 2: keep in mind grub versions 1.9 and above are grub2. if you run easyBCD you want to choose grub2, not grub legacy.
multi-booting Sabayon 5.5 kde 64 bit and others
intel e6600 @ 3.0 ghz core duo
6gb ddr2 800
biostar tpower i45 motherboard
nvidia 9800 gt
3) 7200 sata's non-raid