zee,
When you download, the system asks YOU to pick the folder and directory where it will be stored.
I suggest that you put it somewhere easily located.
I created a Download folder on my C drive specifically for downloads.
You can look on your hard drive under the main hd/windows/temp or hd/windows/internet files
Look for the Avast folder.
Anyway, even if you have a set of files in the Temp folder, they can be easily cleaned out.
Do as Technical advised.....download a new copy and save it just in case you need to reinstall it at a later date.
Good luck.