Start With Red Hat Rescue
Red Hat has excellent documentation, and if this is RH EL its support is paid for at least 30 days past installation. Red Hat also has a good rescue disk. It would not hurt to try to restore from that.
What told you you had viruses?
I would also contact avast, and bring their support into this. Is this the home edition or did you pay for it?
A live CD -- I have forgotten if the Red Hat Rescue (CD 1) will act in that capcacity or not -- or a Knoppinx CD may be what you have to have to preserve your data and install again, if it comes to that.