Yes there is. I was in exactly the same situation as you were. Here are instructions I followed for Windows:
1) Get QuickPwn RC3.
First Step on the iPhone Hack Journey | iPhone-Hacks.com
2) Get the iPhone 2G Firmware 2.0.2 IPSW. Note the filename!
MyTopFiles - Search for mirrors
3) Get bootloaders BL-39.bin and BL-46.bin.
4) Place your iPhone in DFU mode and connect it to your PC with the USB cable.
5) Extract the QuickPwnRC3.zip you downloaded in step 1 and run QuickPwnRC3.exe.
6) Select iPhone 2G (the one on the left), click the next arrow.
7) Click the browse button and specify the iPhone1,1_2.0.2_5C1_Restore.ipsw file you downloaded in step 2. Wait for it to load, then click the next arrow.
8) Check the checkbox next to "Unlock phone". The rest of the options are up to you. Click the next arrow.
9) Here's where you specify the BL-39.bin and BL-46.bin files you downloaded in step 3. Click the next arrow.
10) Follow the instructions on the screen when prompted. This is where you'll need to press the home and power button in various combinations and durations. You can release the home button when the jailbreaking process starts. This should take less than 2 minutes.
11) Wait for the iPhone to boot up. BootNeuter will automatically execute and select the correct bootloader. Don't touch the screen. You should see a message saying "Don't Interrupt!".
12) After a minute or two, you will see a message saying that it will take up to 2 minutes to flash the baseband.
13) Once the baseband has been flashed, the iPhone will return to the home screen. Your 2G iPhone should now be fully jailbroken and SIM unlocked!