No info at all... what version of Windows, what version of avast! (home freeware or Pro paid for) ? Everything is important. What was the size of first hard drive, what is the size of new one ?
I guess someone from Alwil will be able to give you some info, but first you have to provide some useful info about your system configuration. Also, tell us what other security related programs have you installed on that system. In this particular case, maybe it has nothing to do with your problem, but anyway, no harm to list all those programs...
Cheers !