It's not apparent to me that WArchcnv.exe has anything to do with converting MHT files. There is an operating System file of that name, though - see:
http://www.fastfixerror.com/whats-wa...t-a-virus.html
You may have installed a trojan...
Furthermore, as Office 2000 isn't supported on Win 10, neither is any application designed for use with Office 2000 likely to be supported. If you're still not deterred, check the usage instructions from wherever you downloaded the file.
AFAIK Word 2002 & later can all open MHT files without the use of a converter; not sure about Word 2000, though.