It is finally possible to get a near-zero latency keyboard on a PC! It's excellent to have, but does require some tweaking. Herein I give you what worked for me, so you can hopefully get what you want quickly, avoid a fair bit of fiddling, and most important, never get that uncontrollable urge to throw your computer out the window.
A little background
To put things in context: my old setup was OK: I had a decent, but old sound card - a Creative Labs' Sound Blaster X-FI Titanium HD “Audiophile Sound Card”. It has a in-house Creative Labs' ASIO driver. which gave me about 20 ms latency - noticeable at first touch, but consistent and I could live with it; I knew of nothing better in a windows platform. I had tried the on-board sound with the ASIO4 ALL driver, which gave me the same or greater latency.
However, the X-FI Titanium & Creative Labs driver started to give me grief - it would freeze my computer at random times - being intermittent it took a long time to track down. Likely the card / firmware was too old for Windows 7 or had an intermittent hardware glitch.
Looking over the rather scarce options - there aren't many sound cards out there these days - I saw the Sound Blaster Zx advertising "one millisecond latency", for $125 Cdn.
Blow me down - The SB Zx does work
The card really does work - my venerable copy of DPC Latency checker v1.2.0 reports a latency peak of .170 milliseconds. It has 5.1 outputs, and a nice clean sound. It meshes well with my Kontakt 5 synth player. It was easy to set up, with one important tip: clean out all of your old SoundBlaster driver's and software first. I didn't because some of the old software packages were useful. Big mistake, that. ;)
It makes my jammer keyboards very nicely responsive: press key; instant sound. The actual latency is circa 2 ms: very tight.
The only problem here is that it make me want to upgrade my portable jammer setup's laptop to the same responsiveness. Alas, this is problematic for reasons that are due to the quirks of USB (USB's is designed for high throughput, not quick response) and the way laptops are made. 'Twil be worth a separate posting if I find a solution.
There is one inconvenience ...
There is one problem if you are trying to use a single SB sound card for everything: Kontakt 5 & the SB-ASIO driver apparently grab exclusive use of the audio card (think of it; to get microsecond response this makes sense). Windows messages, sounds and the internet browsers simply can't get a sound in edgewise with this cozy couple. They eventually give up, issue an error message or just plain freeze up.
My solution: keep the old motherboard based sound system going, and plug a small pair of stereo speakers into the motherboard. Windows and the others will happily send messages to the on-board sound card. If one does not have a pair of speakers handy, I imagine that you might be able to combine the outputs from the 2 sound systems with a Y-jack.
I hope this helps.