Weird soundcard/keyboard/mouse conflict

Using Win XP Home SP3. I just did a soundcard transplant to replace two cheapies with a better quality one (Soundblaster Live CT4760). One of the reasons I chose the new one is that it had two separate line out (front and rear). I wanted to use one for the PC speakers, and the other as an input to my hifi (previously I was splitting a single line out signal). So I connected PC speakers to the front jack and all was going great until I tried connecting up the hifi and the rear out jack. Disaster - mouse cursor disappears and keyboard stops working. I'm using a wireless desktop with a USB transmitter. Switched off, rebooted, tried again, same problem. IIRC I also tried connecting the PC speakers to the rear out with the same result; that was when I gave up and went to bed. So I'm back to a split signal from the front out jack again and all is fine. I tried searching the web and found a few hundred pages about problems with no sound from the rear out jack, but nothing about conflict between rear jack output and keyboard and mouse. There's no sign of any interdevice strife in device manager. Does anyone have any idea what on earth is going on here? Hpefully,

Andy