Computer to Receiver help?

[Talyn]

Hello,

I'm currently working to hook my fiance's computer up to my old Denon Receiver but I'm having difficulty with getting any sound. I'm using a 3.5mm into RCA plugs. For some reason no sound is coming through. Might I me doing this wrong? Or might there be a bad sound card involved here? Or a setting I need to change on the receiver? I'm a little lost and I wasn't gonna stay up all night last night tinkering around. I figured to ask the experts around here. She did tell me her sound on the computer has been finicky lately, like her headphones and crap computer speakers. I haven't finished hooking up all the speakers as of yet, so far the front left and right and center are hooked up. Any thoughts?