Warning! This procedure will wipe the /data partition of your phone.
-fastboot package: http://www.multiupload.com/BMJ24IL6XY
-32/64bit drivers: http://www.multiupload.com/0XZV1LXRVA
2. Go to: http://unlockbootloader.sonyericsson.com/instructions
3. At the bottom click 'Continue' and enter your IMEI (type *#06# to find out), name and email address
4. You should now receive your unique unlock key
5. Extract fastboot.rar and open fastboot folder
6. Hold SHIFT and right click on the background of the folder
7. Select 'Open command window here'
8. Connect your phone in fastboot mode (turn phone off, hold menu button and plug in USB Cable)
9. In the cmd windows type in fastboot.exe -i 0x0fce getvar version
10. If a value is returned, it means the phone is correctly connected
11. Type in fastboot.exe -i 0x0fce oem unlock 0xKEY and replace KEY with the key provided in step 4.
12. You're device should now be bootloader unlocked
If this tutorial is too much for you then itskapil wrote a step-by-step guide (and recorded a video also):