We techs always recommend that you keep an antivirus program functional on your computer. Like all good things, too much can be a very, very bad thing.
Just like mixing various prescription drugs, mixing more than one antivirus can have disastrous effects on your computer. The problem is not in the file scanning portion of the antivirus, but its true, hidden bread and butter, the real-time protection. These functions check everything you open and manipulate for virus-like behavior.
Now, all you have to do is imagine two programs competing tooth and nail for access to everything your computer does and you can just imagine the crawl your computer goes through. I’ve seen it before. Just thinking about it again gives me the creepy cold chills of a truly scary book, without any of the enjoyment. That or the sick dropping feeling in my stomach.
So, as a note, please, please, please, only keep one of any type of active security program on your computer. Only one antivirus program with active, “real-time” protection, only one antispyware program that provides proactive protection. One good one is more than enough.