Hmmm. I don't think he 'revived' him any literal sense - maybe it's something like Light's hologram, an semi-AI of him merged into the virus itself (I mean, think about it, they say all geniuses put a piece of themselves into their masterpieces ...) That way, Wily can keep an eye on Zero and basically have a front row seat for his 'revenge'.
You're right, Wily could have become another avatar/elf/electronic life form like Light's. (Of course, that won't work for ZL, since I used the Weil and Dr. Cain method instead... d'oh!)
I figure the reason Wily helps out Sigma during the wars is so he can create basically a giant trap for Zero to fall into (and he eventually does, in X5).
Agree with this too.. Wily's using Sigma and Sigma's using Wily. Could make for a very interesting short character piece, hmm...