I usually use at least 2 different antivirus programs to verify results if I am uncertain. I use McAfee at home, Norton at work. If I ever have reason to doubt one, I go online to someplace like PcPitstop.com and have them run their antivirus setup. They have 2 to choose from, the standard is McAfee and the other is Panda. Its totally free, although it might take awhile.
The reason for this is twofold.
1) All antivirus companies update their sig files as quickly and as often as possible. Some get there faster than others though. One might have it today while another might not get it till tomorrow. Running two scans cant hurt.
2) Some viruses target specific antivirus programs. Due to a bonehead slip up by a former boss, I had to deal with the Klez virus awhile back. It targets Norton antivirus by finding and altering the sig file so the program overlooks the virus in scans, thinking its harmless. I had to use McAfee to find it. Not a knock against Norton, its not their fault they are the most popular AV company out there.