1) First go in to your tools folder where you put your SDK 1.5r2 (C:\android-sdk-windows-1.5_r2\tools as reference)
2) Take the files adb.exe fastboot.exe and AdbWinApi.dll (found here -
http://sharebee.com/16a06ebb) and place them in C:\WINDOWS\system32
3) Download the Nandroid Backup and Restore GUI made by Markazeh - Here
Extract the folder to your desktop or where ever you can easily find it (I put mine in the SDK folder for easy access)
4) You will need 4 files: Boot.img, System.img , Data.img, and Recovery.img
Place those 4 files in the same folder as the Nandroid Recovery GUI folder you just extracted in step 3
5) Boot your phone into fastboot mode (Turn off the G1 and turn on while holding the BACK button. If you have an older SPL then hold the CAMERA button and press POWER)
6) Once in fastboot (you should see a white screen with three androids on skateboards) plug your phone to your computer via USB. You might have to press the BACK key to turn it into FASTBOOT mode, or follow the on screen prompts.
NOTE: If you dont have the drivers installed please look one post above to setup fastboot
7) Open NANDROID_Recovery_GUI.exe, everything on the left hand side should be "found!"
8) Choose an option from the right to Restore All, Restore all except Recovery, or Restore Data Only. Let the program do its thing.
Once its done you can reboot and all your info from your backup should be restored!