Like Zigar said, most of the time you get a virus sent to you, the sender doesn't know it. You would be amazed at the number of times I have had to contact other companies to get them to clean up their systems (6 different companies because of SirCam). Normally they are very thankful, but sometimes they try to deny it came from them (until you contact their isp and get a copy of thier router logs )

Always be wary of any executable attachment, if it is .exe, .scr, .lnk, etc etc. Many virii are changing their executable names, and also have many different options for the email message. Many are now comming from people you know, so just because your sister sends you a file, don't assume anything. Always verify it. Call her, or ask send her a message asking if she ment to send you the file. Finally, don't always trust your av software. My company got badtrans and goner both sent to us atleast 24 hours before I saw the first warning, and 36 hours before I was able to get an update. Fortunately, I have gotten my end users well trained.