Dell Inspiron 15 function key issues

I've had this laptop for about 5 years now—mostly running original Windows 7 Home Premium—and it’s been pretty solid. Recently, I did a fresh install of Windows, also original, but in my rush, I didn't get an antivirus set up immediately... ended up picking up some garbage software that broke my Fn key.

So, I went ahead and wiped the entire drive, partitioned everything, reinstalled original Windows 7, and grabbed all the drivers from the official site including the Dell SupportAssist, but the Fn key still isn't working. Now I'm stuck wondering if it's a hardware issue with the keyboard itself, if that malware actually caused damage, or if I'm just dealing with some weird, exotic glitch...

What I've tried so far:
- Installing Dell SupportAssist in Program Files x86 and Program Files, multiple reinstalls, the whole nine yards
- Setting the HID service to automatic start

The Fn key won't work with any combination—can't toggle Wi-Fi, Bluetooth, mute audio, adjust brightness, nothing. What gives? I haven't moved any files over from an external drive, so this was a totally clean install with fresh drivers, yet things are somehow worse...

Are you positive you installed the correct driver? (The ATK package)
Any chance some liquid might have spilled on part of the keyboard?

http://ccm.net/faq/18976-acer-aspire...ey-not-working

It doesn't matter if it's One; if you have Dell SupportAssist installed, you're stuck.

I followed that exact guide—did everything by the book. Nothing spilled, nothing leaked onto the keys. I actually took the whole thing apart and cleaned it out two months ago, and honestly, I barely even touch it now... it mostly just sits in my bag. If I’ve used it more than five times, I’d definitely know. I’ve already tried every single "fix" Google keeps throwing at me.

As for the drivers, I grabbed the official ones from the Dell website. Usually, once I reinstall, everything just works. Which other ones am I supposed to be messing with?