ClamAV. It's free and open source, plus it can be used for corporate use. It's one of the very few free antivirus programs that can be used in a large corporate office. The only other one I can think of is Comodo, but that one has a problem updating sometimes. Just check their forums.
Anyway, the problem with ClamAV, and it's Windows counterpart ClamWin is that it doesn't provide on-access scanning. That means you don't get real-time protection when a virus or another piece of malware tries to infect your computer. You generally have to wait until the next scheduled scan for ClamWin to find the infection. By then, it may be too late.
Not anymore. There is a cool free companion to ClamWin that provides on-access protection. It's called Clam Sentinel. From their page:
Clam Sentinel lets you scan files for malware with ClamWin Antivirus in real-time as they are added, modified, or copied to the computer. It scans permanent hard drives and USB/removable drives with ClamWin, but Clam Sentinel also uses its own system monitor to detect unknown malware that does not have a ClamWin signature. Sentinel is designed especially for older computers, such as Windows 98, ME, 2000, and XP, but it has been tested on Windows Vista/Windows 7 machines and works fine on them too.If you have a small business with ten or less employees, I would recommend using Microsoft Security Essentials because it's free for business use for 10 or less workstations. If you have more than 10 people though, but still can't afford the high cost of commercial antivirus, then perhaps you should give ClamWin with Clam Sentinel a try.
What antivirus do you use at your office? Is it a free solution or an expensive solution? Let us know in the comments!