Lead isn't needed. the "radiation" TEMPEST (Van Eck interception) uses in electromagnetic radiation, ie radio waves, not particulate, ir "radioactivity". TEMPEST proofing is an extensive process and can get very expensive. It really isn't worth the trouble unless you are doing something that certain specialized government agencies would be especially interested in - in such case you have bigger problems.
To block such eavesdropping, you have to start by securing all EM fields inside the computer case, the keyboard, the monitor, mouse, KVM and all connected peripherals. You then need to securely shield all cables said equipment is connected to and interconnected with - this includes securing and filtering the power mains. This must be done for every machine on the network. Moving outward, the rooms for such systems are configured as Faraday cages to prevent signal leakage out of, or into, the room.
Oh, and the secured network does not connect to the Internet, except through several more secure layers.
After that you move to physical security ...
For some more info along these lines do a search for "Sensitive Compartmented Information Facility" (SCIF).