epox pent 4 overclock?

As the title suggests i have a Epox 4g4a pentium 4 that i just built not more than a couple of months ago and i have Win XP as my operating system.This bundle was bought from overclockers and had godd reviews regarding clocking a 1800 to a 2.4 This is what my pc is running at the mo or to be more accurate 2.39.when fist put together it used to run fast and did everything at lightning speed.At the moment it seems to have slowed down and sometimes i find myself waiting for my pc to respond.In truth its not very slow but it is noticebly slower can anyone help with speed problem.thanks to any responses.