Install Personal Home IP Widget on your N900
Install OpenSSH on N900.
You will be promted to enter a password during installation.

After that, you add the Personal Home IP Widget to your N900 Desktop and connect the N900 to your WiFi at home.

The Personal Home IP Widget will show your IP address.

Enter this IP address in PuTTY on your PC and you are good to go!

WinSCP (to see all your N900 files on PC over WiFi) should work the same way!