I just use openoffice/excel.
All you need to do to take stats for arena is to write down your total number of wins, losses, and (optionally, but highly preferred) turns to win (TTW) and EM rate. I do that by putting together three columns on a spreadsheet for each deck. Column 1 is Win/Loss, where I enter 1 for a win and 0 for a loss. Column 2 is TTW, where I enter TTW shown on the victory screen for every win (and leave it blank for losses). Column 3 is EM, where I enter a 1 for an EM and nothing for a non-EM. If you set it up like this, your win rate is (sum(column1)/count(column1)), your ttw is (average(column2)), and your EM rate is (sum(column3)/sum(column1)).
If you don't want to set up a spreadsheet (though it only takes like 30 seconds to set up), you can track stats with something as simple as a piece of paper where you just write down "W-7", "L", "W-9", "W-11-EM", "L", "W-10", etc, and then either do the math when you're done or even just go ahead and post the raw data.
edit: If you need more help setting up a stats tracking system, say so. I can post more detailed instructions if you want to use the spreadsheet version and aren't familiar with operating openoffice/excel.