It's not necessary to get a premium virus scanner.
I don't have an anti-virus on my PC at home, and it's been fine. There was this one time I got a few ad-ware viruses on it, but they're gone now ^-^
Some free programs you can use are these
Malwarebytes, JRT (Junkware Removal Tool), and ADW Cleaner.
They're all free. JRT is the fastest and easiest to use.
When you scan with Malwarebytes, it will ask you to restart your PC to finish. You can delay it for later though.
When you scan with ADWC, it will force your PC to shut down when it's done. So use it as the last program you're gonna use.
If you ever suspect you have a virus, use JRT first.it will scan your PC, and let you know what it found/deleted for you.
If that didn't work, use Malwarebytes. It takes a bit longer, but finds more stuff.
And if that doesn't work, use ADWCleaner. It takes about as long as Malwarebytes, but does its job!
Good luck, and be careful!