Q: Should I run more than one anti-virus program?
A:Short answer: no.
Long answer: Anti-virus software to be effective must hook in to the deepest levels of the OS and even modify how the system works. Often employing the same techniques root-kit and Trojan programs do to secure control. So having two applications competing for supreme dominance over your PC is not a battle you as a user want on your computer. I humbly suggest installing AVG Anti-virus or NOD32 and using an on-line scanner like Housecall for a second opinion.
One other note. Stay away from Internet Security suites from any vendor. They are always bloated and ultra prone to conflicts. Unless you are a Slowsky turtle slowing the PC down in general is not a good thing.