Before I explain why, let's get some definitions out of the way. The term antivirus has come to mean a program that launches when you boot your PC and stays running in memory, protecting you in real time not just from viruses (which are, technically speaking, passé), but Trojans, rootkits, and all other forms of malware.
Keep in mind that every program running uses RAM and clock cycles, potentially slowing down every other running program. A well-made antivirus program has a very small footprint, and doesn't slow things down significantly. But two such programs running together will slow it down twice as much.
And it could be worse. The two programs may conflict with each other--remember that every time you download a file, both will try to scan it. Conflicts could result in other programs failing to work and Windows becoming less stable.
If you're worried that one antivirus program isn't enough, you can augment it with an on-demand malware scanner. Unlike antivirus programs, they don't hang around. You load one, update its database, scan your hard drive with it, and close it when you're done.
I use two of these programs--the free versions of SuperAntiSpyware and Malwarebytes Anti-Malware. Once a week, I scan my hard drive with one or the other.