On your windows 7 computer you will need to set up a shared directory. I only have Linux so I don't know how to do this off the top of my head, easy in XP - I guess it's similar, but more frustrating in Win7 : )

On the N900 install wizard mounter and put in the appropriate settings for your Windows 7 shared directory.

Once you click Mount in Wizard Mounter, the shared directory will then be available through the file manager on the N900.

Apologies for the lack of detail, hopefully this will get you pointed in at least one working direction. I believe others have had success with ssh server on the N900 and winscp on Windows.