ffmpeg and multi threading

I have 4 big wmv files i want to convert to avi.

I use ffmpeg to convert them.

I notice the ffmpeg uses over 90% of CPU (all cores) when it is running in background.

I then create two threads each running an instance of ffmpeg. The CPU usage now reaches 100%.

Should I leave the max threads at two then as its using 100% CPU.

I once tried 5 threads (5 instances of ffmpeg running at same time) but the PC started bleeping and slowing down. I take it this is due to overloading the CPU?

Is your CPU multi-core ?