Anyone know a nice, free, reliable program that can get rid of viruses?

I'm pretty sure I have some because my computer seems to be getting slower and slower + earlier it was being hella glitchy and weird. I had McAfee from the time I bought my computer but it was just a damn trial so now it's over and expecting me to pay up, like ch henny goodbye you weren't even useful. But yeah I don't know a thing about which virus protectors are actually good so I need some info.
