A few methods to set the Samsung Galaxy S to Download mode (from XDA website by Golmar88)
FIRST
1) Power phone off
2) Plug USB into computer
3) Hold vol up + vol down
4) Plug USB into phone while still holding the vol up and vol down
5) Yellow /_\ (triangle, Downloading, Andy shoveling, Do not turn off Target!!! symbol)
Your now in Download mode
SECOND
1) Power phone off
2) Plug USB into Computer
3) Plug microUSB into phone
4) Hold pwr + vol up + vol down
5) When Batt icon goes away, release pwr, still holding vol up + vol down
6) Yellow /_\ (triangle, Downloading, Andy shoveling, Do not turn off Target!!! symbol)
You should now be in Download mode.
THIRD
1) Power phone off
2) Pull battery out
3) Hold vol up + vol down
4) While holding vol up + vol down, plug in USB cable into the phone
5) While still holding vol up + vol down, put battery back in
6) Yellow /_\ (triangle, Downloading, Andy shoveling, Do not turn off Target!!! symbol)
You should now be in Download mode.
FOURTH (adb)
1) Enable debugging on your phone (MenuSettingsApplicationsDevelopment)
2) With phone on, plug USB into computer and phone
3) Open cmd
4) Type in “adb devices” (not totally necessary, but I always do it just to make sure everything is good), making sure it recognizes your device
5) Type in “adb reboot download”
6) Yellow /_\ (triangle, Downloading, Andy shoveling, Do not turn off Target!!! symbol)