When you plug the phone into USB, you should get a USB notification at the top of the screen - drag down the notifications bar and select "USB Connected", then hit the "Mount" button.
If you then browse to "My Computer" or "Computer" you should see the G1's micro SD card listed as a removable device, and you should be able to simply drag and drop files to it. It's a good idea to rename the SD card to something else such as "G1-SD" in windows explorer.
Alternatively, eject the SD card from the G1 and stick it in your laptop or card reader and copy the files.
Cheers,
Wolf