I fear my computer is ill but the virus scans disagree....

**shorter version below**

So, my computer has been working steadily slower for multiple weeks now. I scanned it repeatedly over this time (McAfee and Malwarebytes), but all scans came up clear. It now takes 2-3 minutes to shut down, and if toi close it while it's trying to shut down, it won't. When I start it up, it shows only the screensaver. I hit the power button to turn it off, and this worked previously. This time I tried and it wouldn't go off even then, and when I finally got it off, it a dit it didn't shut down successfully. Before, it ignored anything had been wrong. My computer is only a little over a an old, and I'm worried....

Since that was quite long, if toi don't want to read it: Does anyone have a suggestion for a really good virus scanner/protection?