How to downgrade kernel?

[Jonathan Melhuish <lists@netacclaim.co.uk>]

Just a real quickie - hopefully so quick that it would be more effort to 
flame me for not RTFM! ;-)

I've recently bought a Realtek-based WiFi card (a D-Link DWL-610).  
Despite having a linux driver, it seems to have a pre-compiled 'secret' 
part that is compiled for kernel 2.4.18 (only!).  Bloody closed-source 
crap... >:-(

Anyway, at the expense of being locked to a specific kernel version, I'd 
really like to get it working.  So I use apt-get to download the 
appropriate kernel, headers and pcmcia-modules.  But how do I "activate" 
it?  Presumably I need an initrd image in my boot partition?  None have 
been forthcoming.

Cheers,

Jon