Re-installing lilo after the XP installation is all you should have to do, except from possibly adjusting it a bit before. Just make sure you have something (for instance the Slackware CD) to boot Linux from before you've got lilo back in business, since XP will overwrite the MBR.
I would do like this...
1) Install XP. :P
2) Reboot into Linux using the Slackware CD.
3) If something in lilo.conf needs to be adjusted, edit it directly or run liloconfig.
4) Run lilo.
Done.