A lot of them are viruses. I know well enough to just not open them though. It's quite obvious that theres a huge possibility of attachments from random people being a virus. It intrigues me how so many people open stuff like .pif .scr or whatever attachments, any attachments from a person you don't know. Are people really this dumb? Apparantly :P

IMO, virus scanning software isn't enough. You need latest anti-virus software AND firewall's to stop hackers. I have had hackers attack my PC through open ports completely unknown to me, it's exceedingly easy for a hacker to take control of any PC connected to the web and run stuff on your PC without the need for them to trick you into running it, like email attachments. They can do it all by themselves.

Microsoft Windows XP has a built in firewall which blocks all incoming stuff on every port. It doesn't block outgoing stuff, but obviously incoming is the major threat. It makes your computer completely invisible to the internet.

There are other firewalls which block outgoing stuff to. Unless you run something from a hacker on your computer though which sends stuff out, like trojans do, to block outgoing is not necessary. So just don't run anything from random people.

I use Norton Anvi-virus 2004 and WinXP firewall. I used to get a lot of attacks without the firewall, but have not had a problem since I enabled it.

Don't know about other brands, but Norton's anti-virus has an Auto-protect feature which scans every file that you run, before opening it. Have both these things and your PC should be pretty much 100% protected.