I hope this site is helpful:
http://tldp.org/HOWTO/Linux+WinNT-2.html
Anyway the way I understand your post is that lilo is still there is just that WinXP does not appear on the boot menu. I guess this is possible if you installed WinXP on a separate hd and it didn't overwrite the MBR. If that is the case when u boot into linux which I guess you are able to, as root:
cd /etc
add the following lines to lilo.conf
other=/dev/hda1 (replace 1 with whatever hd windows is installed)
label = WinXP
then run /sbin/lilo
This should add the WinXP to the boot menu. If on the other hand u have destroyed lilo the web page should be more helpful although the previous suggestion of installing winxp first than linux is probably easiest.