Thanks, I really appreciate your help.
Again, the situation is exactly the same for me. My PC is also new and I bought MS-Office 365 online. So it was a piece of cake, to uninstall and re-install Office. However, the first time round I forgot to turn off Avast. And in the middle of the installation, the moment ONENOTE.EXE landed, Avast reported it as a malware and quarantined it. I made the silly mistake of trying to restore it while the installation was still going on, but it didn't help.
So, one more round of uninstall and re-install with Avast turned off from the get-go, did the trick. Now, to make sure the problem does not resurface, I have added the file as well as the directory it is in, in the exclusions for a Quick Scan and the Full Scan. I have also submitted the file as a false positive, though I am not sure if that has worked. It appears that Avast wants to know the Version # of the file being submitted, and for the life of me, MS Office 365's interface is pathetic. There's no more the "Help" -> "About" menu.
Goal is now that they come up with a fix, so that I can remove the file and directory from the exclusions list.
Again, many thanks to theProtoMartyr for your help.