There are 2 software-only projects that could run on N900

• The standard USB/IP Linux project : http://usbip.sourceforge.net/
  To use on N900 you need to have Power Kernel and the usbip package (sudo apt-get install usbip) either installed.
  If you want to use N900 as USB Server you need H-E-N package also
• The USB Redirector project: http://www.usb-server.com/index.html
  This is a commercial software but only Windows Usb Server must be purchased (if you need it), Windows/Linux Usb Client and Linux Usb Server are free to download and use with only one restriction: If you use the Windows Usb Client over a Linux Usb Server unfortunately you must pay the commercial license.
  To use on N900 you must download the armel version (prebuild binaries with sources for kernel module) and try to compile the tusb.ko module for your kernel.

For the first project there is an old thread here on TMO: http://talk.maemo.org/showthread.php?t=41870

The first project supports Windows also but only as client.
If you want to use Windows as Usb Server (and for example your N900 as client) you need the second project (and you must pay the Windows commercial license)