In my case, wanting to install Ubuntu 10.04.4 alongside XP and finding the alongside option missing, the underlying problem was a fixable disk error.
This was revealed on a detour through GParted LiveCD, which can be used to resize partitions. It would only resize in combination with moving the partition, and flagged a problem in a yellow triangle with a "!" in it.
and say "Y" to the question about running chkdsk at next boot (it can't run on the drive you are logged in to).
Reboot and let chkdsk run. It can take some hours.
After that it was possible to use GParted, which no longer flagged an error, to resize (shrink) the partition XP is in. (If you do this, leave the rest of the disk un-assigned, don't partition it.) Do the resize/shrink after defragging the disk so you can see how much space XP really needs.
After shrinking the partition and on running the Ubuntu installer again the "alongside" option re-appeared. I took it, the install completed, XP survived, and it's defaults have seem to be fine.
It might be worth trying the install after fixing but before resizing, but do defrag.