Hello, and welcome to LQ.
First of all, I'd like to ask for a bit more information on the problem, namely:
How did you install grub ? Because slackware installs lilo by default.
Where exactly is this error, and post the exact error plus a few lines before. It needs to be exact, because just "invalid magic number" at some time during boot can mean many things.
Please do provide other info that might be useful, like what you might have done before this error to maybe inadvertently cause it.
Think about it from our point of view, you give us two phrases to go on (of which only one contains information), to diagnose an error like this ... what do you think the chances are of us doing this (omitting psychic abilities some of us may possess).