*Please only follow these steps at your own risk etc etc as I cannot be held responsible if anything goes wrong!*
I searched for ages for the best way to do it and I think I've managed to portray it here in the video. A noticed an old thread which someone did on here (in the sticky section) has now been 'taken off' so thought I'd create a new one.
Let me know if you have any questions/comments and I hope this helps some of you out!
I've also done some quick instructions which should help out along with the video:
Today I will be showing you how to get your HTC Desire S to be “S-ON”. You only need to follow this video if you want to revert your phone back from a S-OFF state.
Why would I possibly want to do this?
There are several reasons:
1) If you need to send your phone back to HTC for repair it’s probably a good idea to remove any software you’ve put on the phone — they may no like it
2) If you want to revert to how your phone was when you first purchased it this is the best way to do it (some people call it factory defaults)
3) You’re bored and have 18 minutes and 5 seconds to spare.
What you will need:
1 x HTC Desire S (with at least 50% battery)
1 x USB Sync cable
1 x half-cup of tea
1 x de-stressing device
Firstly, you will want to download an RUU file. RUU stands for “ROM Update Utility” and this file contains the original software/firmware for your phone.
Go to here: http://forum.xda-developers.com/show....php?t=1002506 and then click the link to FileFactory which will bring up the list of ROM files. Find the one that matches your phone. If you are in the UK like me you’ll see “UK” in the file name. You also need to select the correct file depending on the phone operator/network you are on. I’m on T-Mobile so went for the “TMO” version.
Whilst that downloads, on XDA Developers again, search for “Revert back to stock (remove revolutionary)” http://forum.xda-developers.com/show....php?t=1387484 and you can download a lovely package from “giby21″ which contains Android Flasher and the HBOOT file you need.
**PLEASE NOTE** The RUU file for your phone is quite possibly different to the one mine uses so you may need to use a different HBOOT version. If you don't have the correct HBOOT version you will get an error messagee from the RUU setup/installer saying "error  bootloader version error".
Extract the files and then load Android Flasher. Please note you must have HTC Sync UNINSTALLED, USB Debugging mode enabled (in your phone’s settings) and you also need HTC’s Fastboot drivers installed for your phone to be picked up later. You can get the fastboot drivers from here: http://unrevoked.com/rootwiki/doku.p.../revolutionary
Now select your device in Android Flasher and the select the Browse button under where it says “Bootloader”. Browse to where you extracted the “Revert to stock” .rar file and then the folder called “FlashENGHBOOT”. Select the .img file in there (named engHBOOToverARX.img) and press OK.
Now power off your Desire S and power back on whilst holding the Vol Down button. Click to select FASTBOOT and then plug it in via USB. Now back to the Android Flasher program and select “I’m already in fastboot” checkbox and press the “Flash!” button. You should now notice the REVOLUTIONARY title at the top has gone — great — it’s been removed.
Once flashed, reboot your phone and leave it plugged into the USB. Now you can right-click the RUU file you downloaded and “Run as Administrator”. This process will take about 10 minutes but after this you will be back with S-ON and to your original T-Mobile (or other operator) software.
Phew! Time for a drink!