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Depends how much you want to Sync and how much you just want a vaguely up to date copy.

First you need to install Firefox on your Mac and then use File==>Import to import your Bookmarks from Safari.

Then you can use this: http://www.google.com/tools/firefox/browsersync/

or just go to the bookmarks menu==>Organise Bookmarks, then File==>Export, and copy that file to the PC side and re-import it into Firefox.

Of course this depends on using Firefox on your windows partition.
 
If you use Firefox FoxMarks will do it.

This is what I use. It keeps my bookmarks perfectly syncd among: Portable FireFox installed on a thumb drive, FF at work, FF at home on XP and FF on my macbook.

And if by some odd chance I blow up a computer.. my bookmarks are always saved.
 
Another vote for Foxmarks.

Export your current Safari bookmarks to Firefox, then install Foxmarks. Any computer you boot up that has Foxmarks installed will be automatically updated with all your bookmarks. IMO a "must have" extension.
 
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