When I maintained my client's computer few days ago, I found his computer infected by trojan downloader Win32/Kuluoz.B
These trojans infected my client's computer since he clicked on the fake email claimed from Federal Express.
As a matter of fact, the delivery companies like Federal Express, UPS, DHL and US Post Office never send email notification. They don't know your email. If they cannot delivery package, they will left a note at your door.
It took me an hour to clean up and re-scan whole computer to make sure it clean. So, beware when you check email.