Xperia T freezing on boot?

[JedWareham22]

My girlfriend has an Xperia T as have I. She bought hers a few months ago and a few weeks ago her phone turnt itself off and when we switched it back on, it froze. I used previous knowledge of how to soft reset the battery/phone and it came back to the bootscreen and got through 2 sequences of the "wave" animation and then froze. Turnt itself off and tried again and would continue to do this repeatedly until it ran out of battery. We have tried the soft reset countless times and have tried to do a hard reset (3 vibrates) the hard reset switches the phone off completely and stops it from booting itself. 
She took it into a phone shop and they said they didnt know what to do and another said it was unfixable. I am unsure whether they legitmately know that its unfixable or whether they just didnt have the decency to admit they didnt know how to go around it. 
I have tried hooking it up to a laptop however all that happens is the phone starts its booting process and freezes again. I dont like the fact that she did nothing wrong with the phone and it just stopped working. Any help would be appreciated.

[Thommo]

Try performing a system repair as this will often fix issues - This will factory reset your phone

Switch off your phone and unplug from Pc (Hold volume up and power for around 10 seconds)
Start PC Companion and select Support zone then Update my phone/tablet then in Blue Repair my phone/tablet and follow the onscreen instructions - When prompted connect your phone still switched off holding volume down or back button - This should start the repair or reformat process

If you are using Windows 8/8.1 or a 64bit operating system then adjust the settings for PC Companion and run in compatibility mode and chose Windows 7 or XP

[JedWareham22]

I have tried however every time I hook it up to my computer, it turns itself on and starts trying to boot on and off and therefore the installation process on the computer is always stopped