Why is my CPU Usage at 100%?

My Notebook was running smooth as silk until about a month ago when I now get huge lag spikes and terrible mouse response times when I am doing nothing on the computer except looking at task manager showing my CPU level at 100% . I have a Asus 60x with 4GB of ram and plenty of Memory. My programs start to not respond randomly and i am pretty sure it is because of my 98-100% CPU level. I have scanned for malware and viruses and nothing is found. Please help. Thanks