Need to repartition my external HFS+ formatted hard disk for use on Windows

Hi there

I formatted my 500Gb external hard disk as HFS+ in Tiger when I got it a few months ago, but realise that it would be good to be able to use this with my partner's Windows PC to transfer files/backup her system. So I went to DiskUtility and found that I cannot reformat my disk without deleting everything on it. This is not an option for me as the data on the hard disk is larger than any other storage I have at the moment. Is it necessary to have to reformat the disk in order to get the PC to use the disk?

The hard disk I got came with MacDrive test software, which was a temporary solutions. But as this software costs just about half of what I shelled out on the hard disk in the first place, I'd rather not have to go and purchase this in order to solve the problem. I'd rather just buy another hard disk (very reluctantly, I may add).