Downloading issue

Hopefully there's somewhere out there that can point me in the right direction!

I'm running two laptops from home. My partner has a Sony and mine is a MacBook Pro.

My internet is provided by the dreaded Hughes.net, which sets a daily download limit. If one goes over this, it puts you on a 24 hour go-slow (about 3 times slower than dial-up, making it practically unworkable). This has never been a problem until recently when I've been repeatedly getting hit with it. I've checked my download history and the amount has increased to levels that I know I've not used.

My first reaction was that a neighbour was perhaps stealing it. However, I live in a fairly remote place, my nearest neighour is about 1 mile away and only usually there at weekends, so this is unlikely. To be sure, I bought a new router and set a new password. The fact that the levels are up on an hourly basis and not getting hit during a short period also leads me to this conclusion.

After numerous tests, I've come to the conclusion that it is only downloading these high amounts when the Mac is online. When the Sony is solely online the amounts are very low, around the amounts you'd expect for the usage.

As soon as I put the Mac online, up goes the usage to massive amounts.

I've run ClamXav, MacScan 2 and iAntiVirus and nothing is showing, yet it's still showing huge download amounts.

Sorry for the long-winded explanation!

Does anyone have any ideas?

Many thanks.

James