I'm running FC4 on a HP Pavillion zv5000 (laptop). When installing my wireless USB mouse worked perfectly up until the sound card configuration, whereupon it stopped working at all. I'm getting the signal to the USB end of it so distance isn't an issue.

The mouse works fine at the login screen and until about 5 seconds after I'm logged in and the session is being restored. I tweaked around with restarting and a reinstall and again it worked fine once I was logged in, until I changed the audio settings (switched from Autodetect to Thread Open Sound System since the autodetected hardware couldn't play back). Once I applied the mouse ceased to work. The touchpad works well enough, though.

I'm very new to Linux as could be expected.

From xorg.conf
-----
Section "InputDevice"
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/input/mice"
Option "ZAxisMapping" "4 5"
Option "Emulate3Buttons" "yes"
EndSection
------

the problem is in what KDE is thinking about what is supporting that sound card. Try changing that setting back to Autodetect and tell us what is going on with playback exactly.

Error while innitializing sound driver:
device: default can't be opened for playback (Invalid argument)
The sound server will continue, using the null object device.

Some sounds will play on autodetect though. The mouse will work after logging in sometimes also, but will cease after several sounda have been played.

It is possible that you are experiencing an interrupt conflict.
Perhaps cat /proc/interrupts may give a clue.