everything is playing at the same time?

I'm not sure exactly what you've got hooked up, although I'm leaning more towards a reciever/amplifier?

The computer should be providing a 2.0 signal, same for the DVD player's analog output, and the VCR should be capable of either mono analog out, or stereo analog out (depends on age)

If the reciever/amp device has multiple channel inputs, the devices need to be connected to stereo inputs, not all to one 5.1 input set.

Other than that, I have no honest clue what exactly you have setup there, so I have no honest clue how to help you with it. What it sounds like though, is that its all running on 1 5.1 input and you're using a standalone amp which is designed to be run with a preamp/processor that would provide input sorting, what you might consider is getting one of those cheapo RCA switching boxes from radio shack that take 4 or 5 stereo pairs in, and output 1 stereo pair, and setting that up in place of a preamp/processor