Free Antivirus Software

I’m using a nice Dell 390 Precison Workstation with an Intel Core Duo processor that a client loaned to me. I’ve set it up to boot into either 64 or 32 bit Wïndows XP Pro, or Vista Business. I didn’t want to buy antivirus software for all these installations, but I didn’t want to be without virus protection, so I looked around for free antivirus software that didn’t have the restriction that it could only be used for home or non-commercial use. I found PC Tools Antivirus. It works great on Windows XP. It works on Vista too, but it gets blocked at startup and I couldn’t figure out how to selectively stop Vista from blocking this one program. Each time I boot, I can tell Vista to let it run with a few mouse clicks, but I got tired of doing that every time I restarted Vista, so I went looking for another solution.