Assuming you have the two attached to the same Ethernet network...
Enable sharing on the PC, and then Command-K from the Mac. In a few moments you should see the PC's name come up as a server, choose it and log into it using your Username and Password on the PC (not the one you use on OSX). Then you have read and write access to your drive.
If you ahve a firewall on the PC, you may have to enable file sharing to happen.
First, I recommend you empty your trash and clean out your internet cache before you do a backup.
Then mount the Windows share.
Then tar the file to the windows share. Just pop open a terminal Applications->Utilities->Terminal). I would change to root just in case, but you can probably do it as yourself (ie without the sudo in the line below):
sudo tar cvf /Volume/Share;Name /Users/MyName
And then it ought to ask you for a password. Tarring the directory will avoid problems with filenames not copying correctly.
This is off the top of my head, but I think it should work.
Excapt that will slaughter any resource forks you have. Perhaps rsync would be a better choice there? Or use gnutar, which I think has a flag for preserving resource forks. Assuming you have files that rely on them..