Problem: My actions (mainly just web browsing) online seem to be causing high amounts of internet data usage, on everything I use!

Story: Internet provider starts charging me overages for going over my monthly data limit. I find that out of 4 other people using our WiFi network my MacBook and I are the root cause. ( up to 1gb of data every hour w/o downloading a single thing is what I'm producing)

I decide to erase my hard drive and start fresh hoping the problem does not occur. But it continues so I take it Apple and using their Wifi network the problem does not seem to occur.

Stop using my MacBook altogether and begin to use brothers iPad (for web browsing, twitter, Hotmail, gmail)
Data usage spikes up again this time from the iPad.

I'm currently using a desktop PC with Microsoft and today it started to spike. I'm not sure what's haunting my online presence but everything I touch on my network seems to start spiking, mainly "packets received" when I observed the activity monitor on my Mac.

My suspicions: Trojan Horse or Virus of some sort from a pirated torrent of Office I downloaded to my Mac
Trojan or Virus from BackTrack5 - downloaded to my Mac and Avast Scan gave me a warning on this file, as well as another file "boot.root loader" that was in my "Resources" folder.
Any one of my online accounts is compromised in some way: twitter email accounts, maybe even my apple ID ( passwords were all changed after my visit to the Apple store)
FlashBack Trojan from Adobe, I recently downloaded Adobe Acrobat Pro Trial recently on my Mac before this started to happen and Apple has dedicated a section to Adobe on their security page

I'm really stumped here guys... are viruses able to latch onto online accounts of mine and follow me wherever I go?

Tomorrow I plan on downloading the anti-Trojan detection software apple recommends and seeing if it detects anything. My next step is to start deleting accounts one by one starting with my Hotmail since it has the most traffic and potential of a virus coming through.

I know I just wrote novel so any advice or tips are greatly appreciated.