Hi,

Did you create a recovery media after you set up the computer for the first time?

When you upgraded the OS to 8.1 Pro, Did you just upgrade the OS or wiped everything off and started over again?

Thank you

I decided to upgrade to a pirated Windows 8.1 PRO and I totally regret my decision.

Why?

See here:

http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/download-windows-8-1-retail-and-oem-iso/

Nope, I didn't.

I just put the serial key, I didn't wiped out everything.

I tried the NIR SOFT that looks for the product keys, I saw the WINDOWS BIOS OEM KEY, so that's the original product key of my laptop?

You can probably upgrade to the latest version of Dell Backup and Recovery and then revert to the factory image:

http://dellwindowsreinstallationguide.com/dell-backup-and-recovery-1-8-1-71/

What if I try to reinstall Windows 8.1 Single Language again? Since my laptop has the BIOS OEM KEY ?

Will it work?

First try reverting to the factory settings using Dell Backup and Recovery as instructed above, it should restore your Windows 8.1 Single Language. If that doesn't work then you can download a Windows 8.1 Single Language .iso and clean install:

http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/download-windows-8-1-retail-and-oem-iso/

Alright I'll try reverting it first, if it doesn't work I'll reinstall the Windows 8.1 single launage iso. I'll update when it works,  thank you for helping.