Advanced steps to take for Kies connection problems

1. Ensure your phone is not in USB debugging mode:
Go to Settings, Applications, Development and make sure USB debugging is switched off.
2. Press the Home Key your GS2 and make sure it is not locked. If it is, unlock it.
3. Plug the USB cable into the PC.
4. Plug the other end into the GS2.
5a. Pre KH3: You will now see a picture of a USB connector entitled MTP Application, with a message on the bottom saying Connected Press home key to quit Samsung Kies.
5b. KH3+: You will see an MTP-Initialization message in the notification bar. If successful you will shortly see that message change to MTP Connected.
6. Keep tapping the screen of your GS2 so that it does not shut off (and possibly activate the phone's lock if you have one set). Keep doing this until you get to step 14 below.
7. Wait for Windows to pop up an Auto Play window.
8. Select Open device to view files using Windows Explorer
9. In the window that this opens, double click Phone.
10. Keep this window open, minimize it if you want to. If it remains open for any length of time (10+ seconds) then you have a great chance.
11. Open Kies. As long as Windows Explorer's Phone window is still open then you will probably find Kies detects the GS2. 
12. Kies should now be attempting to connect to your GS2.
13. Once connected you will see your phone appear in the top left of Kies
14. Begin using Kies.


If unsuccessful along any of these step then give it a few attempts. Dont just give up after the first connection failure to do something more drastic. I have had the Auto Play window (point 8) not popping up at all after connection, or it pops up, stays open only for a few seconds, and it closes itself. If this happens, unplug the USB from the GS2 and repeat from step 4.
If reconnecting the cable at the GS2 end keeps failing after several attempts, unplug both ends of the USB cable and repeat from step 2.
If this again keeps failing, unplug both ends of the USB cable and try a few more times in another USB port, repeating from step 2. If it still continues to fail each time then more drastic action is required:


If you can NEVER get your PC to see your GS2
You need to wipe all existence of Kies from your PC and try again. To do this follow these steps:


Open up your PC's Device Manager (right click Computer, Properties, Device Manager) then plug in the USB cable and connect cable to GS2. You may find that the device list refreshes every second or two and the device does not get added. If the Auto Play window pops up, then device manager will stop refreshing and you will see your GT-I9100 appear under the Portable Devices section. If Auto Play does not pop up, Device Manager will continually refresh. This indicates a problem:
1. Uninstall Kies
2. If you were previously on a version of Kies prior to 2.3.0.12035_16_4, run program files\samsung\usb drivers\uninstall.exe (This is important! This does not get done on a Kies uninstall)
3. Reboot
4. Delete the Samsung folder from Program Files.
5. If you want to be even more thorough, run a free utility called CCleaner on the files and registry, and if your confident, remove anything that references program files\samsung in the registry using regedit. (You have to know what your doing with regedit, as it is possible to "break" your PC if you do something wrong).
6. Reboot then reinstall Kies.
7. Plug in your GS2.


Do not rely on Kies to detect new version, as it will only detect major increment leaps.
It will ignore the last number, and will only detect a change if the first group of numbers in the version have changed. (See this thread)


Personally I think that all these problems we are having do not lie with the Kies software itself, but with Samsung's USB drivers for the device, as I've seen the Auto Play window disappear after a few minutes when not even opening the Kies software, thereby constant refreshing in Windows Device Manager occurs. I dread to think what would occur if this happened in the middle of performing a firmware upgrade!! 


Just remember, plug+play your GS2 isn't. It's more like plug+pray!


If you simply cannot connect to Kies at all, then try the following:
1. Open up your dialer
2. Dial *#7284# This opens up the PhoneUtil menu


There are 2 sections:
UART
MODEM
PDA


USB
MODEM
PDA


3. Ensure PDA is selected for USB. If it already selected to PDA, try reselecting it by selecting Modem, then PDA.
4. Press the Back key
5. Retry Kies


Download the latest version of Kies and manuals here


Kies Version History
2.3.1.12044_18 10 (May, 2012) Current version
2.3.0.12035_16_4 (6th Apr 2012) 
2.2.0.12014_18_8 (8th Mar 2012)
2.2.0.12014_18_7 (9th Feb 2012)
2.1.1.11124_17_6 (2nd Jan 2012) 
2.1.0.11112_41_7 (2nd Dec 2011)
2.1.0.11095_121_6 (9th Nov 2011)
2.0.3.11082_152_4 (24th Sept 2011)