If you click a link (from an email) you give that line off commands permission to run on your computer. It could just be something innocent like loading a web page in your favorite browser or something more harmful, loading a malicious program like a key board logger and opening a web page.
Your chosen ID at the moment doesn't matter then as a key logger only will register (and send) the keys that are used and the order in which they are used. Normally your protection program should prevent it from running anyway, or find it with the next scan and delete it. In the same way a virus could be installed on your computer. Most rubbish that comes in that way is caught by your firewall and virus scanner, but some do get through.
So clicking email links can always bring a risk and if you don't know the sender you might as well try to get to the recommended web site by typing it in your browser by hand. Your browser program is much better suited to block unwanted lines from running then your email program like MS Outlook.
Windoze is coming out this week (10 September 2008) with a nice patch just to prevent that kind of tricks. So keep your machine updated and think before you click.
