my OS becomes so slow within couple of days

Hi, starting from 8th july (where as i have remembered) I'm experiencing slow down in my operating system, I did not install any program during last week. However, yesterday I cleaned system from "ccleaner" and then used "malwarebyte" to remove any possible malwares but did not notice any improvement. I'm using "kaspersky 2007" and also have scanned whole system but did not find anything or any improvement. So please help me out about this problem. Thanks for all your cooperation.