[Sony Xperia P, U, Sola, Go] kyzn:
This is a working fix for the Xperia P Flashmode/Fastboot on Windows 8.1.

Disclaimer: I'm not the original author of this fix. Use it at your own risk.

All credit goes to Samyak Ranjan over at Droid Now. LINK

Note: Please double check in Device Manager if you have Intel eXtensible Host Controller under the Universal Serial Bus Controllers. It's listed as Intel(R) USB 3.0 eXtensible Host Controller.

Note 2: You will need to have a USB 3.0 and a USB 2.0 ports for this to work. USB 3.0 for the fix and the 2.0 to install the S1Fastboot Driver.

Here are the steps:

1. Install the XP default drivers included in the Flashtool. It's in the FLASHTOOL\DRIVERS folder in wherever you installed it.

2. Download the attachment I have included in this post. It's the official Intel Drivers for USB 3.0 Driver: Intel® USB 3.0 eXtensible Host Controller Driver for Intel® 7 Series/C216 Chipset Family. I've already included the modded files that works for Windows 8 and 8.1.

3. Disable Signing Reinforcement in Windows. Hit WINDOWS + C, go to SETTINGS ---> PC Settings ---> Click the Search button ---> Type in "advanced startup options ---> Click "Change advanced startup options" ---> Click the button that says "Restart".

4. After it reboots, choose Troubleshoot ---> Advanced options ---> Startup settings ---> Click Restart ---> Hit 7 on your keyboard (Disable driver signature enforcement). Your computer will Reboot again, log in as normal.

5. Open Device Manager.

6. Go to Universal Serial busses at the end and double click “Intel USB 3.0 eXtensible controller”. Click on Drivers in the New window which opens.You should see that the Drivers are provided by Microsoft.

7. Now click “Update Driver” and then select “Browse my computer for driver software”. Next choose “Let me pick from a list of device drivers on my computer” and select “Have Disk”.

8. In the Window that pops up titled “Install From Disk” choose “Browse” and navigate to the location where we replace the attachment.

("...\Intel(R)_USB_3.0_eXtensible_Host_Controller_Driver\Driver_Installer\Drivers\Win7\x64\") Select iusb3xhc.inf and click ok.

9. Ignore all warnings and install the drivers. After installing, Reboot the computer with the same procedure as Step 3 & 4.

Note: At this stage any devices you have connected to USB 3.0 hub will cease function. So fair warning. They will be fixed after you finish.

10. On opening the device manager, you should see an Unknown device. If you already had one before, you need to cross-check every unknown device to find the one we need. Double click each such device and go to the 3rd tab “Details”. Find the Value “VID_8086″. If you only have one listed unknow device it's that one even if it does not have "VID_8086".

11. After finding the correct device, choose “Update Driver” then click “Browse my computer”, Windows will ask you to identify the type of device, scroll down and select “Universal Serial Bus Devices”.

12. Click next, choose “Have Disk”, “Browse” and select the modified iusb3hub.inf that we placed in the same folder earlier. Ignore all warnings and install the drivers.

13. Restart the computer normally and look at the driver tab for the “Intel(R) USB 3.0 eXtensible Host Controller” and the “Intel(R) USB 3.0 Root Hub” to confirm that you are now running the Intel drivers instead of the Microsoft ones.

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Now plug in your XP while holding down, the S1Boot Fastboot Driver should now install. I've double checked it via Sony Update Service and Flashtool 64. Both now register my XP.

EDIT: If the file is corrupted, here's the Mediafire link. Click!