Well about a month ago I bought a Windows 8.1 64 bit laptop. When I tried to install the driver for my USBasp programmer, windows complained about it not being a 'signed driver'. I researched the web and found a rather tiresome and time consuming procedure to temporarily disable the requirement for 'signed' drivers, and again tried it and it worked fine. I'm not very competent with Windows procedures so I can't talk you through the whole process, but can only report that it is possible for to get a USBasp programmer to work with windows 8.1 64 bit, so don't give up.
Upon rereading your posting, why are you trying to select a com port for a USB programmer? You just select the programmer type and then you can only do two things with the USBasp programmer from the IDE, either the "burn bootloader" option from the tools menu or the "upload sketch using programmer" in the File menu.
Only the more 'standard' method of upload sketch uses a com port to upload using a com port to an arduino board/chip that contains a bootloader to except the upload from the IDE via USB serial com port. 'Programmers' use ISP protocol to upload to a board via a six pin ISP connector.