This is usually because of the initrd driver. Make sure you specify on the boot line "-r <ramdisk-name> root=/dev/ram".
The other possibility is that you don't have enough RAM to boot from a RAM disk (you really need at least 6 MBs with recent kernels). In this case, it is possible to write the ramdisk's contents to a high density disk and try booting from this (but, of course, you do need a high density drive to do this). Ask in the newsgroup for help if you don't know how to do this on your own. Alternatively you may be able to get someone else to compile a smaller kernel image specifically for your system (which will save a lot of RAM).