TCPdump
If you're running a Linux based wifi router, tcpdump might be available on the router, which would be ideal. Otherwise, run it on the problem PC and log the results to text file.
You didn't mention whether this was for long term analysis or troubleshooting, so it's difficult to say how much storage you'll need, but tcpdump is pretty efficient and by generating text files, compression will work well. You can also filter very efficiently with tcpdump to minimise the volume of data you are storing.
Biting the hand that feeds IT
