...the thing about memory and its speed is that some computers don't seem to mind as long as they can accomodate it or slower. Your pc2100 [MAY] work fine, if a little slower - but you probably won't notice that outside of hi-fallutin' games.
The other thing to note is that some motherboards will not work with certain BRANDS of memory.
Try it and see. You won't do any damage by putting it in. If it does not work, try going to the Crucial site and asking them what memory your MB needs.