1. Pull up a commandline, go to Documents and Settings\username\.android directory, and do
echo 0x04e8 >> adb_usb.ini
2. Install KIES. (Downloading the separate driver package didn't work--the driver package refused to install.)
3. Set device in adb mode (Settings | Applications | Development | USB debugging(x)).
4. Plug device into computer. You will get a tray icon with stuff spinning. Click on it. If things go fine, you'll eventually see a list of drivers with "Ready to use" and green checkmarks.
If all the drivers have checkmarks, you're done--you had it easy. This guide is for cases like mine where you find that two drivers, "CDC Serial" and "SAMSUNG_Android" are labeled "No driver found" and adb doesn't work.
5. Right-click on My Computer, choose Properties, then "Device Manager". Expand "Other devices".
6. Right-click on "SAMSUNG_Android".
7. Choose "Update driver software" | "Browse my computer for driver software" | "Let me pick from a list".
8. Make sure "Show All Devices" is selected and click on "Next".
9. Click on "Have Disk" and type in "C:\Program Files (x86)\Samsung\USB Drivers\25_escape" (auto complete made it easier to type; if you have 32-bit windows, the " (x86)" should be omitted).
10. You will get a two-pane selection of the drivers. Selecting one of the two "SAMSUNG Electronics Co., Ltd." listings (you may need to look through both) in the left pane gives an option list on the right that includes two "SAMSUNG Android ADB Interface".
11. Choose the first "SAMSUNG Android ADB Interface" and click on "Next". You will get a warning that Windows cannot verify compatibility. Accept this. Windows will install the driver and once it's done you'll click on "Close".
12. Windows installed the driver and ADB started working. (If it didn't start working, I would now have gone back to the Device Manager, right clicked on "SAMSUNG Android ADB Interface", and then redid steps 7-11, but this time with the other "SAMSUNG Android ADB Interface" driver.)
This is enough to get adb working. But it's still probably worth fixing the CDC Serial device.
13. Repeat steps 7-11 in the Device Manager with the "CDC Serial" device now, except that instead of the ADB driver you will look for and install "Samsung Mobile USB CDC Composite Device" in the right pane in steps 10-11.
If all goes well, you will get more spinning stuff in the task bar, and eventually Device Manager doesn't show any devices in "Other devices" with yellow triangles. But if things go for you like they did for me, you'll have "CDC Abstract Control Model (ACM) in "Other devices".
14. Repeat steps 7-9 in the Device Manager with the "CDC Abstract Control Model (ACM)" device now. (You may have some USB Modem options showing up in some places, but I ignored those, and did the same "Have Disk" move as before.)
15. I got a single-pane display showing only "SAMSUNG Mobile USB Modem". (Maybe you'll have two-pane display and you'll have to find it as in steps 10-11.) Select it and click "Next". I didn't get any warnings. Windows will install the driver and once it's done you'll click on "Close".
At this point, Device Manager showed all devices correctly functioning. And "adb shell" worked fine.
16. If you don't want KIES, just uninstall it, but make sure not to uninstall the drivers (the KIES uninstaller will ask).