You could just format your hard drive, find MS DOS 6.22 on some vaporware site, then install it before you install XP. During installation XP will see the older version of DOS and create an entry for it in the bootloader, so you can choose whether you want DOS or Windows at every boot. You could also install an older version of Windows like 95 or 98, and XP will dual boot them just the same.
Or you could trash Windows, install Linux, and use dosemu to run all your old apps.