[Guide] How to root Sony Ericsson Xperia X10 on Android 2.3.3

The Sony Ericsson Xperia X10 has been updated to Android 2.3.3 Gingerbread. If you do update you will loose your root, so you can follow this guide to re enable it. The X10 The device features a 4 inch touch display, 1 GB of internal storage, an 8GB microSD card, an 8 megapixel camera and is powered by a 1GHz processor with 384 MB of RAM. Requirements SEUS or PC Companion installed on your system (for drivers). Phone must be on Stock SE 2.3.3 firmware OR custom 2.3.3 ROM. Modified Flashtool Package by DooMLoRD. It also contains 2 Firmwares which will be needed in process. Download it Here and then extract it in folder wherever you want (from where you can run it). Phone must be on USB Debugging mode ‘ON’ , Stay Awake ‘ON’ and Install App from Other sources ‘ON’. Other Requirements Pause/Stop Antivirus software. Recharge your phone to full. This method involves Kernel Downgrade and again Upgrade by means of flashing as other Rooting way is not possible. DON’T CLOSE THE FLASHTOOL, UNTIL THE PROCESS IS COMPLETE.

Process

1. Switch OFF your Phone (Please ensure requirement 4 is checked ). DON’T CONNECT PHONE JUST YET.
2. Launch FlashTool by clicking X10 Flasher.exe file. Select ‘FLASH’. A dialog box will open.
3. Select 'aZuZu Kernel Downgrade' from the list and on other side selection should be 'Loader'. Press OK.
   
   
   
   
   
   
   
   
   
4. Now some codes will run and info box will appear asking you to do 4 actions.  You can ignore the first two which have already been done.
5. PRESS and HOLD “Back Key” and now Connect USB Cable.
6. As soon as you will connect cable, it will start to flash the firmware.
   
   
   
   
   
   
   
   
   
7. Wait till flashing is done. DON’T CLOSE FLASHTOOL; PLEASE!!! And don’t follow those instruction said. Proceed ahead.
   
   The next two steps are of importance so please be aware that they will need to be done quickly to proceed 
   
8. STEP 01: Be ready with cable. Be ready to be real quick. Switch ON phone. You will feel small Vibration. Quickly connect the Cable back.
9. STEP 02: Device will start but get stuck on SE Logo. Don’t panic and focus on the FlashTool on your computer screen. Keep your cursor ready near faded ROOT Button. Click it quickly, as soon as it appears.
   
   
   
   
   
   
   
   
   
10. The Flashtool will start Rooting your X10. When it is done, your phone will restart. Switch it OFF. Disconnect the cable. (Even if u can’t switch it off, just remove the battery. Just don’t let it switch on fully) If your phone doesn't restart which happens most cases, then remove the Battery directly. After this disconnect your cable.
11. Keep battery out for 10-15 seconds and insert it back. DON’T SWITCH ON THE PHONE.
12. Now in FlashTool select FLASH. In dialog box select; 'aZuZu Kernel Upgrade' and 'Loader' on other side. Press OK.
    
    
    
    
    
    
    
    
    
13. FlashTool will ask you to connect USB Cable like before. PRESS & HOLD Back button and connect the USB.
14. FlashTool will start flashing your X10 again.
15. When all done, disconnect cable and restart the phone. You must be rooted now and have SuperUser icon in menu.

source : XDA Developers