I'd check out the PC to see if it was using HDMI to the monitor. Then I'd setup the usual receiver with that and disable the sound blaster. I'm sure you read http://www.tomshardware.com/reviews/high-end-pc-audio,3733.html so I won't discuss why we know not to stick with any particular sound card.
https://www.amazon.com/Best-Sellers-Electronics-Audio-Component-Receivers/zgbs/electronics/14269301 is the usual. I didn't find any sub 100 buck unit. My bet is that while you can get a fine stereo receiver for this you'll have to go ebay.
Can someone please help me out here? I am looking for a 5.1 amplifier so that I can hook up speakers to my computer. I currently have a full set of speakers but no receiver. I have been scouring the internet trying to find an amp that will that is less than $100.
If it helps, I am going to be hooking up the speaker system to a Sound Blaster Omni external sound card made by Creative. All I am looking for is an amp capable of 5.1 output with inputs compatible with this card. I am not really concerned with what style the output terminals are, as I can solder on appropriate connectors.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic