1st of all - USB mode should be switched to <Modem> - most probably this is the reason it never asked you for drivers when you connected handset and started RSD lite.
Secondly if you connect the phone to USB (without starting RSD lite) it's not necessarily prompt you to install drivers at all, or may prompt to install only few components. Once you start RSD lite these prompts will rain like hell. (LOL, manual says about 8 components - I do remember more than 10

- but it can depend on the driver version though.)
Another hint - do not try to start flashing before all components are accurately installed - my 1st flashing attempt failed namely because of this.
And one more thing - make sure you use Windows XP SP2 on your PC
- I never heard about successfull flashing on Vista
- Windows XP SP1 may lack some generic drivers too (I'm not sure though)
Hope that helps!