Hello priyadarshi, and welcome to the forum.
I suggest that you disconnect your PC's from the network, since the infection is spreading, and that you only reconnect them once cleaned out.
If all PC's are connected during the cleaning process, they may very well be reinfected by one of the others because there is malware that once installed on a machine look for other machines on the local network, etc.
1. How many PC's are you dealing with?
2. What version and product of Avast do you have? 5.0.594 is the latest version.
3. What other security software is installed including firewall?
4. What attempts or tools have been used so far to remove the malware besides the Boot scan, even if unsuccessful?