47 Kudos
Don't
move!

Having a Sony Xperia phone with a locked bootloader makes flashing and rooting your phone a little more difficult and we have to find other ways to get around this. However, it is possible and this is what you want to do.

 

Get your Xperia Arc up to Android version 4.0.4, not 4.0.3, but 4.0.4. Important, you can do this by flashing it manually (Flashboot) or using Sony’s PC Companion to update it.

Now, you’re going to want to flash the firmware back to 4.1.B.0.431 (Ready To Root) with the following .ftf file.  This is for the Arc LT15i, but if you look around the Internet you can find the right files for the Arc S LT18i.

Mirrors:

Sorry, the file is so big (365 MB), but it’ll be worth it.
Use Flashboot 0.9.9.0 to flash the entire system, wipe data, wipe cache, everything. Start fresh.
After that is done, start up your phone to make sure everything is good.  When I did it on mine com.android.phone and Keypad kept crashing. No worries, that will be fixed later, when we flash back to a more recent firmware.
Now that you’re phone is ready to root, you need to run DooMLoRD’s Easy Rooting Toolkit for Xperia 2011 Devices V1
NOTE: There are other versions of this V2, V3, and V4.  However, I have found those don’t work, and that V1 is the one you need.
Follow all the directions and make sure those ADB Drivers for Xperia Arc/Arc S are installed.
After the device is rooted, you can flash another ROM using ROM Manager by ClockworkMod or Flashboot 0.9.9.0.
I recommend one of these Xperia Stock Firmwares if you are using Flashboot, otherwise pick your favourite maybe try CyanogenMod