I thought I'd post this real quick in case someone else is experiencing the frustrating problem I was having. I have a Microsoft Wireless keyboard and mouse 700 and it has been driving me up the wall lately. The keyboard would miss keystrokes, would occasionally respond with a repeated keystroke and would otherwise be annoying. I tried replacing the batteries, re-syncing and restarting and nothing helped.
Then, I tried moving the receiver device that plugs into my USB port further away and voilĂ : Here I am typing up a storm without getting stressed. It is nice to have a keyboard that works again. Apparently the USB receiver cannot be within a certain distance from my keyboard... maybe I should have read the instruction manual.
Update (6/23/10): The manual states that the mouse & keyboard should be between 8 inches and 4 feet from the receiver.
Update 2 (8/10/10): I've officially given up on this mouse & keyboard. The mouse drains a fresh pair of AA batteries in about two weeks and the keyboard constantly has connectivity problems. I went back to old-fasioned wired replacements, for now.