Hm, if Sabayon Linux behaves like Ubuntu, then you have to work manually. An instruction can be found here
- Make a backup of your data!
- Grab a Windows RescueDisk and a Sabayon LiveCD/DVD
- Boot the LiveCD and partition the first two partitions for Windows. You may have to move the Sabayon at the end.
- Install windows on the first two partitions. This will destroy GRUB!
- Boot the LiveCD and repair GRUB.
Sorry, I'm in a hurry right now and the topic isn't that easy. However, it should be done similar to the Ubuntu instruction. If you like, documentate your line of actions. Either here in the forums or in the wiki. Especially with the "dangerous" parts.
It's a difficult topic …