"File does not exist" are 404 errors. That is, someone thinks your web site has a certain page, and it doesn't, the web server records an error saying it found no file to serve.
As for the formmail lines, these are bots or the like of spammers looking for vulnerable formmail scripts. They're just trying out the most likely URLs at which one would reside. Nothing to worry about - unless your access.log shows otherwise. In your access log, you would want to look for status 200 to make sure none of these crackers "got lucky".
The other not found errors are caused by Microsoft software that has extended functions for web stuff. Trying searching the Microsoft site about the owssvr.dll and cltreq.asp files to see what they are supposed to do.
There is nothing you can do to prevent these things from happening.
Of course you can simply put blank files in the locations for the 404s and the 404s will become 200/304s instead and no "file not find" in error.log
You can also re-classify them in access.log - for example, status 202 means "accepted but not able to process", and I use that to classify the formmail attempts. You have to add something to .htaccess to do that.
Perl / MySQL / HTML+CSS