can a virus transfer it self from one PC to another through a shared dsl modem?

Answer 1

generally speaking, no. most viruses require the victim to do something to activate the virus. open an attachment, go to a website, etc. if the virus, say took over your email and went out to everyone on your email list (inclduing the connected computer) that's a possibility. but most likely the other computer is fine. but you should run spyware on both to be safe.

Answer 2

no that cant happen.