@ pranaygtr - As rsvr85 has said, maybe you ought to update to Avast 5.0.377 ....?? I downloaded the final version this morning and it sure is good! Immediately I noticed lower memory use than when I was using version 4.8. The initial "first time" quick/full scans take a bit longer than subsequent quick/full scans ..... and the brand new GUI is great to work with [as compared to the 4.8 GUI!?!] It seems very compatible with W7 too.
Avast 5 immediately alerted me to the Eicar test file I downloaded from here http://www.eicar.org/anti_virus_test_file.htm
and it was promptly dispatched to the "virus chest"!