If he's using slack 9.1, the modinit tools are already included as part of the distro.
It's 2.6.0 "ready".
Now, to Sarno --
Each kernel version you compile will have its own kernel modules. It's likely that you didn't compile everything you needed as modules, or compiled some of it into the kernel.
You see, the kernel has two ways of dealing with drivers... The first method is to compile them directly into the kernel. When you boot, they're in there.
The second way is that it can load them as kernel modules -- inserted or removed from the kernel on a per-need basis.
With the stock 2.4.22 kernel, most everything is done in the modular fashion, making it easy for the one kernel to work on many different systems.
Your old kernel modules are still there, just reserved in /usr/lib/modules/2.4.22 instead of /usr/lib/modules/2.6.0/
2.6.0 needs its own kernel modules, of course.
So, you need to figure out if you compiled in or built as modules all the drivers you needed for the 2.6.0 kernel.