Nevermind, I've figured it out! Here is a step-by-step guide on how to connect your Windows 7 PC to the internet via Bluetooth tethering your N900's Speakout GPRS connection:

1) Connect to Speakout ("Rogers Internet") on your N900 (make sure it's actually using the GPRS connection and not WIFI!)
2) Install "Bluetooth Dial-Up Networking" on your N900
3) Pair your N900 with your PC via Bluetooth (if you don't know how, just download, install & run the Nokia Ovi Suite)
4) Click Start > Devices and Printers
5) Right-click your N900 and choose "Dial-up Networking > Create a dial-up connection..."
6) Enter *99# as the Dial-up phone number and leave the rest blank
7) In Firefox, open "Options > Advanced > Network" and click "Settings..."
8) Choose "Manual proxy configuration", enter "10.128.1.69" as the "HTTP Proxy" and "80" as the "Port". Also check the box that says "Use this proxy server for all protocols" (this will make sure SSL works so you can visit "HTTPS" sites).

That's it! (Again, please remember you need to revert your proxy settings when connecting your PC to the internet via WIFI.)

It should work now. Yay, I'm so happy.