It is very important that the kernel-devel package matches your running kernel. To remove the newer version of the kernel-devel rpm package as root type;
rpm -e kernel-devel-2.6..18-1.2257.fc5
But you need to install the matching kernel-devel for your current kernel-2.6.15-1.2054_FC5 which should be on the installation media or here;
FYI: since FC5 was released a large number of updates have happened, any reason to first update and use the current updated kernel for FC5 which is kernel-2.6..18-1.2257.fc5, notice that this is the kernel for the kernel-devel package you wish to remove. It would be a better idea to update the OS first.
yum update yum
Yes it will take a fair amount of time but it is worth it.