That is why I use Avg and Spybot Search and Destroy. They are both free btw.

Your computer is slow/low on memory so the scan uses up a lot of resources. 

The result is that stuff doesn't happen immediately.  Then when you start clicking repeatedly it piles more work onto the computer making it work even harder and causing it to slow down even more.

Remidns me of my Mcafee Virus scanner. It would scan like every day... for 8 hours.

My computer died because it had like 6-7 virus on it that kept overheating the processor.

THANKS MCAFEE!

I learned how bad McAfee was the hard way too.  A format cleaned it up and a better firewall/virus scanner were definitely in my future.

McAffe sucks, everything slowed down to the point of frustration. I'm running avira windows defender and spybot. And i have a problem with ad-aware while during scans it will reset my computer at random so i pulled that out, i think the program was damaged cuz during an update my dial up internet disconnected.