The DOS partition could easily be the backup/restore partition your vendor cd installs to let you recover/repair if you have a Dell or HP unit...or it could be the small partition that Compaq's used (may still for all I know) that gave you access to your bios for making changes to it.

Deleting the partitions is fairly easy with Delpart.exe (it runs from a regular bootdisk with the program included or you can boot with your bootdisk and then switch floppy disks if delpart is on a separate disk). Once you run the program and delete the partitioning, you can then use your XP cd to boot with (make sure your bios is changed to have the cdrom be the first boot device) and XP can create and format new partitioning and install windows.

LOCATION OF DELPART.EXE

http://www.bootdisk.com in the Utilities area or HERE
Extract the file to a floppy disk, boot up with your windows bootdisk, change to this floppy disk and run Delpart.exe and you can remove all partitioning.