It is indeed possible to create your own file path. Some part of what you are using is not valid. Is there a folder named "my Extract files" at the top level of your C drive?
You probably do not need the text "filewin:" for exporting data, if your version does require a tag, you can use just "file:" and thus use something that is not platform specific.
To debug file path issues, I recommend using the exploration file found here: Exploring the use of a $Path Variable in Scripts
Using it, you can insert a file by reference from the location to which you want to export and then you can use the calculation fields in this file to see the actual file path in order to compare it, character by character to the path you have tried to use in order to detect discrepancies.
With your calculation of "C:\My Extract Files\" as the value of your PCExportPath field, what you are actually passing over to the export script is
"filewin:C:\My Extract Files\&$printfilename which isn't the same syntax as:
"filewin:/c:/My Extract Files/"&$printfilename
you're missing a forward slash after filewin and have backslashes instead of forward slashes in your filepath.
Change the value to "/My Extract Files/" and add the forward slash to "filewin:" and see if that works.