Someone told me that if you use Microsoft Outlook for e-mail, simply opening the e-mail also opens the attachment.
Lots of viruses take advantage of flaws in earlier versions of Microsoft Outlook/Outlook Express (particularly any version prior to 6.0), which allowed certain attachments to automatically open. Microsoft's newer versions and subsequent patches have pretty much eliminated the threat of attachments opening themselves, but there are always those who, for reasons unknown, don't take it upon themselves to update their systems.
Also--hackers are continually finding new (or previously undiscovered) flaws in Microsoft's programs, so one cannot simply accept on faith that not opening attachments will keep you safe. Microsoft security updates are sort of a hassle, but they should not be ignored.
I've been using Mozilla for Web browsing and email for about a year now, and I'm hooked. Also, since I stopped using Internet Explorer/Outlook Express, my computer performance has improved considerably. After I installed IE/OE 6.0 a couple of years ago, I had lots of problems with the computer locking up, especially right at shutdown, forcing me to manually turn off the machine. From the day I started using Mozilla exclusively, that problem completely went away.
There are a few things about Mozilla--mostly the way it displays certain graphics--that aren't quite as pretty as IE, but the trade-off on performance and the fact that it isn't vulnerable to the IE/OE exploits has made it more than worthwhile.