OK, so I know the easiest thing would be to abandon XP, but that ain't an option......



Installing Mandrake 10 from CD onto spare hard drive space (leaving the installed XP partitions alone) went fine, and Lilo booted into Linux OK. But choosing the Windows option from Lilo froze on a blank screen with cursor flashing. And I ended up having to do a clean install of XP, which isn't something I want to do again and again.


I followed all the default options in the Mandrake setup, including the recommended partitioning of the free HD space. Should I be doing something differently?

You don't need to reinstall XP, you can just run 'fixmbr' from the XP cd.
If Mandrake continues to fail in booting XP, you can use the Windows bootloader to boot:
http://www.linuxsolved.com/forums/viewtopic.php?t=125

Make sure your windows addition to /etc/lilo.conf is correct

Solution 1
Solution 2
Booting from the Windows bootloader
Another
A good one too I love the last post.

What you ask has been done many a time before (but there is no clear how-to for dual-drive booting, it seems). These links are all from a quick search I did through threads I have participated in. If you don't find that the answer(s) here are good for your situation, tell us what didn't work (tell what did work as well, as google indexes this site daily, I think, and others will be helped knowing you got your problem fixed).