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

What you are asking about is essentially installing a new Operating System over the existing one.

There is no standard procedure for doing this.

Copy all your important files (My Documents, etc.) to another logical (or physical) drive. Then format the partition in FAT32, or delete and re-create a new partition and perform the standard installation procedures for the Operating System on the newly formatted drive. Once you have successfully installed the OS, copy your files back to the drive.