1) If you can get back to your good kernel 2.4.20, boot into it.
2) From the Slackware folder "patches/packages" retrieve the kernel*.tgz files except the xfs files if you don't need them.
Get the "patches/kernels" folder.
3) Delete the /lib/modules/2.4.21 and the /usr/src/2.4.21 folders.
Delete the /boot/2.4.21 kernel, config, and system.map files
4) Run pkgtool to install, run lilo if running lilo. This will install the new kernel and supporting files without user intervention.
If you go the route of downloading the kernel sources and follow the directions at the top of this forum and don't change the .config
file this time you will get a new working 2.4.21 system.
Don't be fooled the .config is not as easy to mess with as a lot think, there are a lot of dependences and usually not much if anything improves performance as much as you might think.
If you can't resist and must change something, change the processor type and turn off math co-processing.