How can one revert a disk with NTFS to FAT 32 to install older software which won't run on WinXP?

What you asked is actually installation the new Operating System over the existing one.

There is no standard procedure of doing this.

Just copy all your important files (My Documents, etc.) to another logical (or physical) drive. Then format partition in FAT32, or delete and re-create new partition and perform standard procedures of installation Operating System to the newly formatted drive. As long as you successfully installed OS, copy your files back to the drive.