Hi kwikness,
I belive that you could delete the Windows XP partition without problems unless lilo is install in the Windows XP partition.
To make a lilo backup, just copy the /etc/lilo.conf to another device (maybe a usb drive) from slackware.
If something goes wrong, insert the slackware dvd (or cd) and run
# lilo -C /path_to_file/lilo.conf.
Have any doubts, just ask.
Good day