I have just recovered an older windoze machine from the same problem...it was w98, and all I had to do was to boot it so that I got to say y/n to every driver/thing it loads on windoze boot....I left everything concerning the screen off, and got it somehow started so that I just and just was able to open control center & remove all screens...after a reboot it asked me to install a "new screen" and that was it

luckily. but if you can't get it done without a reinstall, then...
you will loose your lilo, if it's in mbr and install a windows. so prepare a boot floppy for your slack, so that lilo can be reinstalled later...then I think you should be able to install windows so that it only formats hda1 where it is installed.
try it... I think you should be fine, as long as you jusr remember to make a bootable floppy for slack. with slack you can then install lilo easily...