Computer downloading massive amounts of unwanted data?

Whenever I connect to the internet from my laptop (Acer Aspire 5100 running Windows Vista Home Premium) it starts to download a large amount of data from the internet. I have not asked it do so but it does it anyway and I have no idea what it downloads. My prepaid internet connection has a data limit of 1GB valid for a month and I try to keep it that way but unfortunately it finishes in about a week. I heard the new iPhone has a similar problem. I did not notice this before because I was using an unlimited data connection but that got too expensive so I switched to the current prepaid one and that's when the problem started. I use the connection mostly to check my email, Facebook and Wikipedia and I seriously do not think that that takes up 1 Gig within a few days. So can someone please tell me how to prevent my computer from downloading all that unwanted data? I hope the problem can be solved without moving on to a new laptop because I really can't afford a new one.

It's not going to be solved by getting another computer. There's not enough information here to figure out the problem. But what I would suggest is run msconfig and shut down any startup programs except the default ones the windows originally used.
In the Run box type msconfig and check "selective startup". A lot of programs put themselves in startup and it is not necessary for them to start when your OS starts.

My suspicions are:
1. The OS is downloading updates automatically. See if Windows Update is enabled. However, I really wouldn't advise this as the updates protect your computer.
2. You may be using torrents. Even if you are no longer downloading, it is possible that your computer is uploading to people who are downloading the same torrent.
3. You have a trojan or worm. Scan with these two:
http://www.avast.com/...s-download
http://www.malwarebytes.org/products/malwarebytes_free