Every web page you view, every email you send or receive, every song you listen to, every video you watch, every time one of your programs (like your antivirus for example) updates, bandwidth is being used and bytes of data are being downloaded. So downloads are not just comprised of programs you’ve downloaded. There’s no way to keep track of every single byte downloaded. There are network traffic loggers you could install but this would be a bit of overkill for your situation and some of these kinds of programs can be expensive.
Your download of 300MB during a 24 hour period seems a little high – unless you were downloading video or large programs. You might want to install a bandwidth meter to check traffic flow during hours when you’re not on the computer. There are several freeware “bandwidth meters” which show dataflow from the Web to your computer and from your computer back to the web. Data coming from the Internet is download; data going from your computer back to the Internet is upload. Every second you’re online there is data flowing back and forth between the Web and your computer – this is normal and nothing to be concerned about.
Here’s a link to a freeware program (1.5MB) called BitMeter. It’s a basic program that displays the amount of data downloaded and uploaded by the hour, day, week and month. You might find this useful – it provides quite a lot of detail, although it will not show you exactly what was downloaded or uploaded. Another program is called FreeMeter. It provides you with a bit more information and has a “URL Grabber” and a log. You can watch the traffic flowing in both directions in real time. You can reset one counter anytime you want – the top counter displays usage for the past day. FreeMeter is a bit more complicated than BitMeter, but it provides more information. Both are free and both can give you accurate information regarding amount of download traffic flowing to your computer from the Internet – and from your computer to the Internet. FreeMeter does not need to be installed, you just unzip the file and click on the executable. FreeMeter is a tiny 43KB download.