You installed the latest Anti-Virus software and checked your computer. Nothing detected, you're safe - right? Inoculating against the measles at best protects you from the measles, but not anything else. Anti-Virus software is the same.
The software checked your computer and found no sign of any virus, but it can only check for what it knows about. Anti-Virus makers know this and update their data files regularly. Many Anti-Virus vendors require you to pay up to keep getting updates. No update, no protection.
Some Anti-Virus software is so intrusive that it barely justifies its existence when it slows your machine to a complete crawl with all its options. Even running these programmes on the latest hardware can make you want to pull your hair out.
Free alternatives exist.
One to look at is AVG, visit http://www.grisoft.com/ and download AVG-Free Edition. Keep it up to date. Keep updating until it tells you that it is up to date - this goes for all Anti-Virus software!
Final tip, in Outlook and Outlook Express, turn off the preview pane - not just hide it(!), because just clicking on an email (to delete it) will open a message and possibly infect your computer. You can do this in the View Menu - Layout/Pane.