It could be a number of things. What is the volume for your rate plan? Have you talked to all the cell phone companies about getting unlimited volume?
Make sure your antivirus is up to date.
I've used ZoneAlarm in the past which allowed one to specify exactly which programs had access to the internet.
One possibility is that you were hit by a number of operating system updates. For example, if you were hit by a Service Pack, although I'm seeing major Windows service packs being released in mid 2011. I think if you go into the windows updater, you can review the update history.
I'd probably make sure my antivirus and antispyware programs were up to date, and install a good 2-way firewall such as zonealarm (I haven't used it for a few years so I don't know about the latest version, but it used to ask one to verify all network activity), then turn off all automatic updates so that you don't get surprised by anything. If you can monitor your monthly network activity, then pull down a few extra updates at the end of the month as capacity permits.
Depending on how rural you are, beware of unsecured wireless networks.
Oh, also install NoScript, and perhaps an ad-blocker. Be judicious about what you open up. This will also knock out a bunch of unwanted video content.