Kies is extremely hit+miss, albeit more miss than hit!
It works almost all of the time on my laptop at home, but I rarely used to get it to work on my work's PC, but it has got slightly better since I done what I've outlined below.
Ensure your phone is NOT in USB debugging mode:
Go to Settings, Applications, Development and make sure USB debugging is switched off.
I find it can succeed on a slightly more regular basis if you plug in the cable, wait for windows to prompt on opening the device (ie. look at the files via explorer). Once this is open, open up the Phone folder and minimize. If this folder remains open for any length of time then you have a chance. Then open Kies. As long as the Windows Explorer window is still open then you will probably find Kies finds the device. If explorer automatically closes that window, then you can give up on Kies, so close it down, and try unplugging+repeating.
GIVE THIS A FEW ATTEMPTS. Dont just give up after the first connection failure. If it still fails, try a few times in another USB port. If it still continues to fail each time then more drastic action is required:
Open up device manager then plug in the cable. You may find that the device list refreshes every second or two and the device does not get added. This indicates a problem:
1. Uninstall Kies
2. run program files\samsung\usb drivers\uninstall.exe (this does not get done on a Kies uninstall), 3. reboot
4. remove the Samsung folder from Program Files.
5. If you want to be even more thorough, run a CCleaner on the registry, and if your confident, remove anything that references program files\samsung in the registry.
6. Reboot then reinstall Kies.
7. Plug in your GS2. Remember, your GS2 is more like plug+pray!
Personally I think there is a problem with Kies USB driver, particularly when other USB devices are connected.