Hey Ladybug,
First let me say welcome to the forum. What I would try first is download a new copy of Avast! 4 Home Edition:

http://files.avast.com/iavs4pro/setupeng.exe

ALSO download the following the uninstall tool AvClear4:

http://files.avast.com/files/eng/avclear4.exe

Then I would save it to my desktop or my documents so it is easy to find.

Then I would uninstall the current version you have on your system by going to the control panel then clicking on ADD/REMOVE Programs. Then wait for Windows to populate the list of programs you have installed then select Avast antivirus and then click remove. Then restart your system.

Once you get back to the desktop then run the avclear4.exe to remove any remaing remnaints of avast antivirus. Then restart your system.

Once you are back on the desktop. Try and reinstall the program. I hope this helps.