What do I need to know so I don't accidentally delete all my files?

Due to a problem that I have with my computer, I was told to use Avast and try to fix it. Upon doing so, I did the custom downloader. I tried using the program, but to no avail. Uninstalled it, and attempted to reinstall again. After a couple of restarts, it still gives me the same message of certain files still being there and me being unable to reinstall.

I check the folder that Avast was installed in, which is my D:\ hard drive, and I see everything I had in it, all 100gbs or so, gone. So, now I’m here waiting for Recuva to save me. Maybe it is because I tried to cancel the uninstaller because I was impatient, my fault. Or I did something else wrong. What should I know so I don’t wipe out my entire hard drive when using this program?

Why do you install avast on D:
What problem is it you have that avast should fix?

If you need help with a virus problem then follow this guide http://forum.avast.com/index.php?topic=53253.0

Attach logs from Malwarebytes / OTL / aswMBR

Bad habit of mine, I guess. I have three HD drives that I can put my files in, and I usually put it in D: over than C: due to more space.

Someone suggested to use Avast for malware scanning, after seeing some errors I have on my computer.

Someone suggested to use Avast for malware scanning, after seeing some errors I have on my computer.
Do you already have a antivirus installed? ..... if so dont install another

Follow the guide i gave link to above and a malware expert will help you when you have attached the logs