I've never used or had MACs or iPods. My mobile uses
Windows Phone 7.5 and my ISP's data account allows
me to hook up other stuff to the internet via that phone's
account and connection. It's known as tethering. I've
only tested it with a Dell laptap of my neighbour when
he asked me to fix it (would not complete any boot).
It just WiFi's in to the phone, gets acknowledged etc
and then it's on the internet much the phone's own
browser. Mobile plan is Unlimited (monthly PAYG).
Sheer guess: WiFi probably. Possibly that Android
tablet or the XP box. Sheer guess YMMV.
sme7admin will give you the 'when' and 'how badly'
on the network use family of graphs. The ones further
down the page give internal and external networ loads.
You should be able to infer a lot of pointing information.
If it turns out to be a windows box then something
called Wireshark (open source) will allow you to squirrel
down through a complete morass of really technical stuff
on your network to give you a closer hunch of where to
search next. Wireshark is quite complex and not very
intuitive but it delivers, though it's up to you exactly
how you interprete what it delivers.
http://www.wireshark.org/