What you propose will not work because your operating systems are not in the correct order, In order to use the windows boot manager you have to ALWAYS install the operating systems from the oldest to the newest except in Vista.

My suggestion would be to download GAG and see if it will pick up the two bootable partitions. It should but I don't know as I have never tried it after the second OS was installed in a dual boot configuration. If not, then leave the XP drive alone and delete the Dos install....not the partition....install GAG (free boot manager) then install the DOS system.