I don't have your exact issue, but I found that after jamming to some backing tracks on YouTube, I wanted to record my improv and practices.
Like you, I was unable to record the backing track + guitar; only guitar would record on PC.
So, what I ended up doing is this. Open whatever recording software you're using for recording, and create a track that records your guitar. Plug in some headphones to your amp, play your backing track, and press record. Congrats, now you've got your guitar recorded on your PC, but no backing track. Bummer.
Create a second track, and insert the audio of the backing track. Sync up the start of your playing with the start of your backing track and give it a shot.