Sorry for my English I like Spanish better.
So you've managed to get stuck in some sort of unforgiven error or logo...
Here's the step by step on how to re-flash your LG GT540 if it isn't getting recognized in Download Mode.
Step 1-. Disconnect your Phone from your PC and pull off the battery, sim card and Micro SD.
Step 2-. Connect the phone to your PC. (You should see a animation.)
Step 3-. Go to your Device Manager (Start -> right click on "My Computer" -> Manage -> Device Manager) [This is for windows XP]
Step 4-. Search for modem, click on the plus sign and then right click on LGE Android USB Modem and disable it. (When asked just press Yes)
Step 5-. Open up your KDZ_FW_UPD.exe (I use the english one) and load it with the KDZ file you want to flash.
Step 6-. Click on Launch Software Update (It should start flashing)
Step 7-. ????
Step 8-. Profit.