In the Save As dialogue box, you'll want to choose a different format, one that the PC will recognise. The default AppleWorks format won't be recognised by the average PC so I recommend either one of the Office formats or something universal like Text (depending on the original file type of course).
If applicable, to preserve as much of the formatting as possible, use the "RTF" format. I can/will be recognized by virtually all peecee text apps, like word, notepad ect, at least back to Win98 anyways