I was updating the firmware on kies for my galaxy s and it got half way through and then said error and is no frozen on the downloading 'do not turn off' screen, I have taken out the battery and put it back in, and tried the hard reset, which leads me to a phone symbol, a computer symbol and an error symbol in the middle, help plzzzzzz
You should experiment with rebooting via the various key combinations / battery pulls / USB plugged-in-ness below first. Sometimes the phone can be jolted out of the bad mode it's in by doing that. If the firmware was partially written though, you'll have to do Odin as Ryan suggests.
I basically agree with Ryan's answer, but I'd use at least Odin 1.5. Here's Odin 1.7, it includes the 512 PIT file.
For the GT-i9000 (international Galaxy S), get firmware from this XDA thread. You probably want the newest for your region (excluding Gingerbread, since it hasn't been officially released).
I also recommend checking re-partition in Odin. This will wipe 100% of your data, however.
I think the easiest way to get into Download Mode is through key combinations. For the i9000, just hold Power + VolDown + Home until the phone reboots, then let go of Power only; after a second you'll get to a yellow triangle with an Android in it. For the Vibrant, remove the battery, plug into USB, hold VolDown + VolUp, and insert the battery.
I had the same problem (except I didn't use Kies, I had the problem after the first OTA Update)
Here are the steps I did to get my device back in a usable state:
Keep in mind, this will revert your phone to a rather old (android 2.1) version of TouchWiz. You may want to see if there are newer ODIN files available.
You will probably need different files anyhow, as the ones I have below are for the Galaxy S Vibrant.
Also, I take no responsibility if this does not work for you, or your phone explodes in the process. These are the steps I took, and they worked for me. Good Luck.
Don't give up if this doesn't work. If you get the device to boot, even if it is to the "Error Screen", there is a possibility of recovering.