Cannot share media file to PC
That is an excellent start but you may still get the same error message as Windows will often want to know the domain of the username (not "Domain" with a capital D).

When you log into a PC over the network from another PC your username will often take the form of:

[PCNAME]\[USERNAME]
[PASSWORD]


So if my pc was called stevespc with a username of steve and a password of 1234:

stevespc\steve
1234


This is so that the PC will know where to get the authentication information from. If you leave off the [PCNAME] then it will assume that it is using credentials from the computer you are logging in from and not the username and password that you want to get to (with a username/password from the target PC).

so you may have to add the computer name of the mac. If you can't recall it you can use the IP address as well. Like this:

192.168.0.1\steve
1234


It's annoying and can be confusing at first but once you learn the trick its no big deal.