I have Word 2000 on the PC at work and Word6 (Mac) on my PowerMac at home. I transfer the PC file to the Mac via floppy, open Word 6 and from that the PC file. NB you might need to get translators from Microsoft to convert Word97 etc to Word 6; (the Mac tells me that a Word 2000 file is really a Word 97 file; and the PC tells me that a Word2001 Macfile is really a Word98...)
Going the other way, open Word 2000 and then the Mac file from that. Watch out for problems with fonts. Word 2000 will print a file transferred from the Mac in the typeface used on the Mac, but it's better to use a font that exists on both systems (such as Arial); you can always change it once you've transferred the file.
The new Mac version (2001) will open Windows Word files without any argument; open the program first and then the file. It also includes the translators.