Hi, I'm also using compaq presario cq42 and have just managed to solve the reversed fn key prob.
press f10 immediately after you on your laptop to enter the bios settings.
go to the system configuration tab &
DISABLE the action keys mode.
This will reverse the fn key mode where:
the f1-f12 key will function without fn
and the dim light, and other ctrls using the fn keys
hope this help