"... but with Win XP and later Windows operating systems, the computer is doing so many things in the background that the Morse element timing will be badly mangled."
I disagree. I use a homebrew interface that combines 2 inputs: 1) a K1EL outboard keyer with Virbroplex Vibrokeyer and 2) the serial port RTS signal to key up my TS-940S. I use Windows XP (latest Srv Pks) and when operating CW, usually have DX Lab Suite, MixW, Firefox and other apps open and operating with no problems whatsoever in the timing of CW sending via the RTS signal. My PC is a 2.6 Ghz Dual Core AMD with 4 GB of RAM. No pain, no strain in my operating CW, up to 40 to 50 wpm (I tested it that fast, NO WAY I could operate normally that fast
Blanket statements like the quote above are confusing to many operators and need to have a caveat added. I will agree that if you have a slow PC and limited memory, you could easily get to a point where the hardware has too much latency to effectively run a digital CW keying interface.