That's because the kernel-desktop-latest-2.6.24.5-1mnb1.i586.rpm doesn't exist.
CODE
[root@apus greg]# urpmq --sources kernel-desktop586-latest
http://mirrors.kernel.org/mandrake/Mandrakelinux/official/2008.1/i586/media/main/release/kernel-desktop586-latest-2.6.24.4-1mnb1.i586.rpm
http://mirrors.kernel.org/mandrake/Mandrakelinux/official/2008.1/i586/media/main/updates/kernel-desktop586-latest-2.6.24.5-2mnb1.i586.rpm
The package you want is kernel-desktop586-latest-2.6.24.5-2mnb1.i586.rpm.
Please update your sources, then do
CODE
urpmi kernel-desktop586-latest