In the Microsoft universe, there is no other operating system than windows. So during an install, while you are reading the bells and whistles this miraculous system is bringing to you, it installs its boot loader, denying access to the rest of your disk. However, all you need to have is information. Before reinstall windows, you have to know what is your root partition. You can see it in the file "/etc/lilo.conf" in the line "root=/dev/hdaX" for ide drives or "root=/dev/sdaX" for scsi drives. Keep this information because you will need it several hours after.
If you do not have the slackware cd, prepare a boot floppy following the slackware installation instructions you did some time ago.
After the installation, you need to boot your machine with the slackware cd or the boot floppy. When you see the prompt
linux ro root=/dev/hdaX
where hdaX must be substituted by the information you kept before the windows installation.
I would recommend you to test this procedure before the windows installation to see if you have all information needed and the diskette reads ok and the system can boot from cdrom, etc, etc...
After boot, log in as root and enter
to reinstall lilo.