Ok, before you try this here is the disclaimer;
If you take it back, they will replace it for you.
If you try this recovery (asuming you can boot into download mode) and it works, there is some chance your phone may not be 100% "right". I have never heard 1st hand of anything getting screwed up by it, but i guess it is a possibilty. If you can get into download mode and this doesnt work, your still dead. It is highly unlikely that they could find out you tried to "service" it yourself and give you crap. I dont know how likely that is, but anything can happen.
This method has worked for me and hundreds, probably thousands of others but there is no garantee it will work.
If you still want to try this, here is what to do:
[STOCK ROM] Odin3 One-Click Downloader and Drivers CAPTIVATE ONLY - xda-developers
here is the abridged version. Download this in your PC:
http://www.thevod.org/misc/captivate/I897UCJF6-final-OCD-REV0.rar
Unzip it and launch it.
Open your phone (while off) remove the battery, the sim and the SD card (if you have one).
Put the battery back in, leave the phone off.
With odin open, your phone off, the USB cable connected to your PC but not the phone yet.
Hold the volume down button, no other buttons.
Plug in the usb cable to your phone.
Odin should detect it (make sure the drivers for your phone are installed first!)
If it has, click the start button.
The rest is automatic. If it has worked, your phone should reboot into it "factory" condition.