I've found out the cause of this: the Mac's HDD were formatted from HFS to FAT therefore, I was able to access it via Windows but not through Mac OSX boot. I tried to repair the volume using Disk Utility but that didn't work out. Using time machine to restore the Mac partition didn't work too probably because the partition is in a different format. So I've decided to back up the whole Mac's HDD partition into a 2TB external HD, format the Mac's partition, then transfer the file back into the Mac's partition using MacDrive. Dunno if it'll work but I'm giving it a try.
Avast! should not automatically format a drive from HFS to FAT just so it could scan it for virus. Even if it does, at least have a warning message to alert the user. Please fix this issue for future users that might also run into the same problem.