|The 100 mb of space if more than enough to install dos on. I've never installed dos along side an NTFS partition but you should be able to boot up with a dos startup/boot disk and use fdisk to partition the remaining 100 mb of unallocated space. Then reboot and format the space. (I'm not sure you can format it from the XP desktop since it's going to be fat16. XP only likes fat32 and NTFS.)|
If that's what you're doing and getting the 'not enough space' message then maybe dos isn't seeing the drive at all. If you're using disk 1 of the 6.22 install set then when it boots up, exit setup and run fdisk and choose option 4. What does it say?
If you get to the formatting stage, be careful. Stand alone dos will probably see the new partition as C: because it doesn't recognize NTFS partitions, although that may depend on what kind of partitions ubuntu uses. But XP will probably assign a different drive letter. So you need to be careful you don't format the wrong drive. It may be best to temporarily disconnect the ubuntu drive while partitioning, formatting and installing dos, just in case.
IF dos sees the 100 mb partition as C: then you should be able to install dos on it. It won't overwrite XP's NTFS partition as that's used space and you'd need to remove the XP partition with fdisk, which of course you're not going to do. But it won't be bootable unless as Stuart says, it occupies the correct sectors or you install a boot loader. I don't think a successful install of dos will interfere with XP but would defer to others with previous experience.
OtheHill's suggestion to instead use a flash drive may be the easiest way to get things done.
"It was far easier for you as civilized men to behave like barbarians
than it was for them as barbarians to behave like civilized men."