I just bought a second processor-identical to the first (E5420) - but once installed, my machine will no longer load Windows Vista Pro 64.
Lenovo support told me that after the error message that I have at boot, there is an incompatibility between the system and this second Intel E5420 CPU.
If the revisions/steppings of the two processors don't match, errors can occur. Be sure to use the latest BIOS. Double check stepping / revisions and P/N ordered.
There was an issue with incompatibility between first and second CPU which was solved in some of the later BIOS versions. Download and apply the latest BIOS available after confirming that both CPUs are the same. BIOS updates may be in 32 bit format - if running a 64 bit system, download the ISO and burn a bootable ISO image on CD-R. Here is the latest version at time of this article - be sure to check for newer versions.