There's no current selection pressure to become 'grey goo'; a virus has no method of replicating itself other than hijacking other cells functions, and then having a mechanism to pass to another host. Viruses are also quite fragile in the environment, most need some sort of fluid transfer between hosts to propogate as they are vulnerable to heat and UV.

A good virus has a symbiotic/parasitic relationship with it's host, the best are highjacked into junk DNA, and don't even have to bother replicating themselves as we pass the sequences to our offspring. Viruses are also fairly specific in what they can target (cell receptors), and if a virus was that aggressive the target hosts would combat with some sort of defence or go extinct.

There is a view that viruses ultimately drive/have driven evolution in a more critical manner than sexual reproduction.