Bulky in the sense what?
There are many improvements in the detection, settings etc. So it is really worth a try.
Although installing over the 4.8 works, sometimes it might cause problems. So,
1. Please download the avast setup file from here :
http://files.avast.com/iavs5x/setup_av_free.exe ,
2. Download the avast uninstall utility :
http://www.avast.com/uninstall-utility3. Uninstall v4 via control panel and reboot to safe mode.
4. Open (execute) the uninstall utility and select avast 4 option in the drop down menu.
5. If you installed avast! in a different folder than the default, browse for it. (Note: Be careful! The content of any folder you choose will be deleted!)
6. Click REMOVE
7. Restart your computer to normal mode.
8. Then install v5.1 using the setup file you downloaded in step 1.