Viruses that attach themselves to executables belong mostly to the past, when viruses were simple and easy to detect.
They hide in various places, even in the BIOS, which means that even the old and trusted method of reformating the hard disk may not get rid of them.
One virus is even a complete operating system by itself, so that it takes over the computer and runs your own O/S as a virtual machine. It corrupts the MBR to boot the virus, rather than your system. That means that no tools at your disposition can find even the smallest hint of its presence, and its use of the network card is invisible to your own O/S.
Today's viruses are the product of organized crime. We no longer have to deal with script kiddies, but rather with organized teams whose budgets may exceed those of law-enforcement agencies. Some estimates put the average banks lose from fraud at 5% of the yearly income. Just try to imagine the enormous sums that are involved and the percentage of this "crime-tax" on the world economy!