I use SuperAntiSpyware the free version that does not have real time protection but if you pay for your copy they give you real time protection you can get it here SUPERAntiSpyware | Remove Malware | Remove Spyware - AntiMalware, AntiSpyware, AntiAdware!
. I just noticed they now have a business version that will work on servers which is great for a big business. I can't tell you the number of times that it has found Trojan horse programs or viruses in its scans. It has saved me a great deal of grief with the free version.