Just solved this one for my situation. I found information elsewhere that your NTFS or FAT32 partition can become marked 'hidden' in this Linux/Windows mess. Using the Partition Magic boot floppies I selected my 'hidden' NTFS partition, went to 'Advanced' and I believe it was 'unhide'.
After that, GRUB would start both Fedora and WinXP with no problems.
Hope this helps somebody else.