It's happened to almost every Linux user at some point: You have a nice installation of Linux, or maybe Linux cross-booted with Windows. So far so good. But then, at some point, you reinstall Windows, perhaps because you upgraded to a new version, or perhaps you just did it to "clean up" Windows, reinstalling the original system files to make it a little more stable. When you reboot, however, you'll find that if you had LILO running from your hard disk's MBR, it has been killed, and Windows has claimed the MBR as its own. How will you reinstall LILO so you can get into Linux?
The answer is simple: You need a Linux boot disk. You can use a floppy or a CD, but whatever you use, you need to type something at the boot prompt to make the kernel mount your Linux installation. Essentially, you'll be using the boot disk just to load the Linux kernel, and after that you will be booting into the Linux installation on your hard disk, just like normal. The key command that you need to type at the "boot:" prompt looks like this:
linux root=/dev/hda1
...Where /dev/hda1 is the partition on which your Linux root mount point is. Once you type this, after the system is finished booting, you should find that you're back in your Linux system. From there, all you need to do is type lilo. This will reinstall LILO, using the same settings you had before. (If you want to reconfigure it, you'd better do that first by editing your /etc/lilo.conf file.)
In any case, once LILO has been reinstalled, you can reboot, and you should find that it comes up at startup again, giving you the choice of which OS to boot.