This is a bit of useful advice to would-be Linux administrators like myself. If you don't run Linux on your PC and don't plan to, this will be of little interest.
The standard way to boot a Linux system is by using LILO, the Linux Loader. One of the things LILO needs to build a functioning boot setup is a mysterious 512 byte file called boot.b. LILO will usually update boot.b but will carefully construct backups of this file; this shows how essential those 512 bytes are.
If you muck around with your system long enough, you may suffer the same fate as I: You will have deleted, overwritten or corrupted your boot.b, and there will be no backup available to CYA.
My own case was exacerbated by the fact that my installation CD was lent out and I had been booting from floppy to begin with and was trying to configure a better boot setup. Some people probably know about this and laugh when they hear I spent a whole evening Googling for a solution. But it was news to me and may help someone else:
boot.b is one of the binaries produced from the LILO source.
Once I found this out, it was a simple matter to download the lilo source
that fit my installation, compile
it and copy the boot.b back where it belongs.