I did the reghack for the 2.2 beta that was available on Kies in September, I've made no other changes - never rooted, no lagfix (could not rooting have been the cause?). I was on some verison of XXJP before and I was updating to XXJPY as this seemed the closest to the one MTN and Vodacom are using. According to everything on XDA you're supposed to flash a CSC too, so I downloaded the collection of CSC files and chose XXJP7 since this seemed the closest. Sorry, don't remember the exact error message but google seemed to indicate that if you flash the wrong CSC then this might happen.
Windows does not recognise the phone at all, tried what you said, tried a dozen other suggestions online (combinations of recovery mode, pressing both volume buttons, battery in/out, cable in/out between these steps - no indication from the phone or windows that the phone is alive.
In trying to enter recovery mode in absolute darkness I can see that the phone turns on with a black screen that flashes on and off if I keep my fingers on the button, similarly it responds with this absolute black screen if I turn it on or put the charger in (and the phone gets warm after some time with the charger in), however no vibration or anything else, just black screen, it goes off if I take the battery out.