Well, I guess the question’s in the subject line, so…
There are a few ways to do this. The first and easiest is: don’t. Macs can handle PC disks and PC files just fine; most softwars made for the Mac can create files in various PC formats (depending on what you’re creating). So save as a PC file (say, Word for Windows) on a Windows/PC disk/Zip/CD, and pop it into your PC.
You can also leave the file in Mac format and save it on a PC disk - you’ll be able to move it to a PC, but you won’t be able to open it. There’s also a program called MacDisk that will let your PC read Mac disks, but again, you won’t be able to open the files.
One of the quickest, easiest methods I have found is via Sony memory stick. It helps that I won a memory-stick reading mouse (like a disc drive combined with a mouse) and that my PC by a lucky coincidence is a Sony Vaio, with in-built memory-stick slot.
With OSX and WindowsXP, many USB devices including memory stick readers are totally plug and play.
So if you have any external devices - such as an mp3 player - that uses memory chips of any format, you might want to try this method. Memory stick readers aren’t too expensive.
Again, if you have OSX, then it will be easier to network the two computers. I’m not 100 percent sure how, as I haven’t needed to try it, but http://www.macfixitforums.com will sort you out.
Thirdly, if the Mac has a CD burner, that’s also a very easy way to swap files.
What’s the real problem here? As I understand it (I use Win32 and *nix mostly), Macs can write to PC formatted disks, so even if you’re w/o a network, that’s not a problem. Obviously, if the file in question is a text file, things are fine. Any image format you care to name, also fine. Executables are obviously out, but everything else depends, IMHO.
I’ll make a giant assumption and say that pretty much every word processing program is as compatible with it’s cross-platform version (if not more so), as it is with the competition’s file format.
If I needed to do this in a hurry, I’d set up a web server or something. OSX has a unix shell, right? Perhaps try scp or something. But it seems you’re somewhat concerned about the file format, so perhaps say what app it’s for?
It is no longer generally necessary to save files in a “PC format” for Microsoft software. For example, for any MS Office product newer than Office 97, the files are compatible with both PCs and Macs. I have worked with “Office 97” and “Office 98 (Macintosh Edition)”, as well as “Office 2000” and “Office:mac v.X”. In each case I routinely e-mailed and transferred files back and forth between PCs and my Mac with no major problems. (The formatting is not always 100% correct, though, particularly with complex documents.)