Apparent abnormal Bandwidth usage ?

I’m on ADSL 384kbps with monthly cap of 1G. Up to recently, this was more than adequate for my needs which are about 2/3 hrs per day Internet usage plus email. I regularly visit sites such as CNBC, Yahoo mail, CNN, BBC and occasionally download mp3 and video files. So far this month (up to 13 October) used 432 MIB of which 340 was downloaded data. I did have Limewire on my pc but have removed it in an effort to reduce involuntary use of BW. I also have Symantec which auto downloads virus def files each day. Have Spybot loaded plus windows updates is activated as is the firewall. I usually leave my modem connected when idle. I’ve run several complete scans of my pc but no threat revealed. Spybot reveals a tracking cookie but says it fixed the problem. Have port locking in place with ISP. I don’t think the problem resides with ISP as recently changed and usage is the same. At a loss to know what to do next to try and identify the problem. Using a piece of freeware called NetMeter which appears to pretty accurate in tracking usage as I’ve cross checked the figs with ISP. Overnight, it showed that 11 MIB was downloaded even though IE & Outlook weren’t open. Can anyone help? Many tks Chris