This may be really simple and I've probably been Googling wrong but...

I've currently got a dual-boot setup with XP on one HDD and Vista on another.  XP is no longer required and I'd like to use that HDD for better purposes but I can't format the partition as it's a system partition.  The Vista Computer Management console was no help and, when I tried running FDISK from a DOS boot disk (downloaded image), it only recognised the floppy drive.

Can anyone help?