Okay now this is case Inspector Clouseau should deal with...
So let me organize it a little:
1)Key boards randomly turn off in the same day or in week
2)You have tried different keyboards to no avail.
My Questions are:
Does it do it when you run certain programs?
Have you updated Windows 7?
Does your mouse or other devices stop working as well?
What are the Specs please? Manufacturer, Processor, Ram etc...
Instead of booting into normal mode boot into Safe-mode (With out Networking) try to see if the keyboard stops in Safe-mode>> This will tell us if its a program or more hard-ware related.
If you boot into normal mode and the keys stop working...Go to Run>Type:cmd>in the black box see if you can use the bad keys...
(if you cant type "cmd" go into all programs and look for it there)
Please answer my questions in order to understand and help you better thanks.