Yes you should have only one Anti-Virus program installed.

But if you want Anti-Spyware, you Should have 2 to 3 installed.
The reason for this is Anti-Spyware works from a database of known malicious programs. Subsequently the Anti-Spyware programs are only as good as there database. The last stats I saw, the best detection for any single Anti-Spyware program was only about 40%. Those number may have changed some, but it's still a good general practice to run more than one program.