You can do it on the N900 and copy it to your windows machine
Code:
ar p *.deb data.tar.gz | tar zx
* does it with any or all debs in the same folder, but this will not work with more deb files in the same dir, because all of them would produce a data.tar.gz file which would be extracted as a folder that would be called the same name. Change *.deb to something else if you have more debs in the same dir.
Try it on the N900, I have not yet got mine in mail so I can't try it out for you yet. If it does not work, try installing easy debian and running it within that.
Copy the file to your memory card (if any), or copy it via network.
