Nah, don't have Mumble installed. Googling it reveals a fair few cases of random crashes to desktop but they mostly seem to revolve around low amounts of RAM and running in a 32-bit OS, neither of which are applicable to this computer. It's a really strange crash - just an instant black screen, monitor says nothing is detected, fans still running, keyboard doesn't do anything (not that I'd bve able to see it if the monitor's detecting nothing, but caps/num/scroll lock lights etc. don't flash on/off). Only way out is a press of reset.
PC gaming is a pretty new thing for me (this PC's only ten days old) so I guess I'll start trying to disable individual bits of software and see if something's decided not to play nice.
The weird thing is that I got an hour's play in after work, then a crash, then the instant crashes every time I pressed Play from log-in. I have no idea what could have suddenly changed at, like, 5:50pm today whilst I was just minding my own business in Queensdale.