Kapsersky Antivirus 2011 uses 98% of CPU when it scans. Why?

Kapsersky Antivirus is consuming the entire CPU when a full computer scan is done. It is extremely heavy on CPU and computer gets slowed down tremendously. I don't know what its doing but its been almost 5 hours but it hasn't finished the scan. I have about 100 GB worth of hard drive (total size is 465 GB, but only 100 GB has been used so far) and 2 GB RAM along with Vista operating system. KAV does not use RAM that much, but its very rough on CPU. It keeps punishing the CPU ruthlessly when scanned. Also I thought that it had a very fast scanning engine but it takes almost 6 hours to scan. I used to have mcafee and that took 9 hours to scan. I also had ashampoo anti-malware and that took 8 hours. But no antivirus is this rough on CPU as KAV. I can't do anything because 98% of CPU is taken by KAV. Even things like opening broswer is so hard. Its very hard to have come to yahoo answers with such slow speed.