Hi,
In this case I don't think it actually makes a difference, but there is a difference between insmod and modprobe. When you insmod you explicitly insert a named module into memory, just giving the module name. Modprobe, on the other hand, consults the dependencies database - e.g.
/lib/modules/2.4.18/modules.dep
(kernel version changes to match yours as per 'uname -r')
It then insmods 'x' number of modules as needed by the one requested, i.e. including all dependencies.
It can be that they are indentical commands because there are no dependencies in loading the module anyway and I think thats the case here so maybe thats of somewhat academic interest other than the fact that the kernel uses modprobe instead of insmod .
Regardless, if you can '/sbin/modprobe 3c509' then its not that anyway.
The suspiscion then comes to something being wrong with your /etc/modules.conf .
Do you have any other modules referenced in there that load correctly ?
Regards