partitioning - Re-installing Windows on MacBook Pro

I have triple boot installed. Everything works smoothly, but Windows is crying for re-install (overfilled registry, too many programs installed). Before MacBook Pro I would typically create another partition and install Windows there, while still being able to access old installation. This is very convenient, since should I forget to copy something from old installation (e.g. settings in some program) I can still boot into old system and copy them over. After a couple of weeks I would then delete old partition and live happy life :).

However, now I can't find a way to do it. On MacBook I have triple boot installed using rEFIt. I combine MBR and GPT (hybrid-MBR) on the drive as following:

GPT Protective Partition - MBR and GPT
Mac OS X - MBR and GPT
Windows C: - MBR and GPT
Windows D: - MBR and GPT
Ubuntu - GPT only
Ubuntu Swap - GPT only

As you can see I already use all 4 MBR entries for primary partitions and cannot create another one more. Extended partitions are not supported in hybrid-MBR. However, I do not really need access to Mac OS X or GPT Protective Partition from Windows. Can I somehow manually choose which GPT partitions are used in MBR, so that I can create another partition and install second Windows in there?

It's known as a "hybrid MBR", and Rod Smith has a detailed guide to creating a hybrid MBR partition table with arbitrary EFI partitions selected, using GPT fdisk. So yes, you can manually choose, with the right tools.