My Parents are on BTOpenworld and get a warning email about usage say 8gb out of 10gb, what's a recommended bit of software to install and track usage so they can see where it's been used say iPlayer, web, email etc.
To be honest anything to do with video streaming iplayer, 4od etc can be heavy going on a limited broadband line. general surfing and emails you are not going to really hit 10gb unless you are on it a far bit.
If you only have the one machine then give this a go.
What OS are you using? If it is Win7 (or even Vista) I would suggest installing the Network monitor gadget available from here. That will be running constantly so that you can have a real time monitor of your network use. If you open settings you can actually setup "remaining usage" monitor and warning....