Why is my computer laggy all of a sudden

[Syabatron]

Today when i came home from school my computer was lagging at random times and it is really getting me mad. It lags at any time, and when i am watching a video, when it lags it freezes the video for a couple seconds then goes back to normal. I have Avast scanning right now to see if its a virus. My build is CPU: Pentium Dual Core E5400
GPU: Geforce GT 520
RAM: 6GB
OS: Windows 7

I dont really play any games anymore except League of Legends which i play most of the time, other times Minecraft or Gmod or average games. LoL and the others play just fine, though i did almost die in LoL because of the random lag.. Well please help...

 

[Brillis Wuce]

1. Restart your PC to see if performance improves
2. Check your temperatures (http://www.cpuid.com/softwares/hwmonitor.html)
3. Open your PC case and clean for dust
4. While case is open, check and verify that every cable is tight and secure
5. Reseat your RAM and video card