Anti-virus shows clean, but programs opening and running slow lately

Hi, I had no problems with my laptop until my nephew was using it two days ago and opened some emails that directed him to an adult dating site. It seems that since then, programs take longer to open, videos stutter, PC games that used to run fine are choppy in single player and have unbearably high lag in online multiplayer, and when I open Task Manager sometimes my CPU usage spikes to 100% for periods even when nothing is running.

I ran Kaspersky and Malwarebytes Anti-Malware checks, but both came back clean. Do I even have malware? Is my computer being used as a "zombie"?

I am running Windows 7 64x. I have a Hijackthis log if anyone wants to see. Appreciate any help!