melodie:
If you cd to /boot then do mv initrd-2.6.33.4-pclos1.pae.img initrd-2.6.33.4-pclos1.pae.img-old before reinstalling the same kernel, it should then make the initrd for the kernel also. If you've already installed a new kernel, you could uninstall the old one through Synaptic, then do a fresh install of the older kernel, to get the initrd image for sure.