Issues with new HiFi playing through computer sound card.

Hey guys,

I recently bought a pair of Mission MX3s and a Yahama AS300 amp for use with my computer. A week or so before this I upgraded my Power Supply and noticed that my computer speakers suddenly developed white noise which changed pitch etc when I moved the mouse cursor. Basically, this issue is still present with my new setup which is plugged in via a 3.5mm to RCA cable. I am wondering if I used the optical out would this remove the noise? Only issue with that is my amp doesn't have an optical connection, so would I need a DAC or something? My sound card is an Asus Xonar DG by the way.

If anyone has any advice on how I can solve this it would be greatly appreciated , thanks.