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1) Just like retrieving any file from a remote location, you should be able to utilise FTP/scp (hopefully not rsh).
2) Depending on what version of PowerPoint you're using/is being used, you should be able to import this with Applixware for Linux. I know 5.0 can import; I'm not sure about the earlier versions. You might also want to check with StarOffice (can be found off of Sun's website). I believe SO can import PPT files as well.
3) If the Win box is behind the IPMASQ, then you might wish to consider implementing NAT to do the address translation. If you can't do it on a software level, then there are relatively inexpensive hardware solutions available (circa $200 or less). Can't go too much into this one, however, since I have only dealt with NAT once.