I downloaded three blender installers for Blender 2.57B. One from CNET, another from Softpedia, and another from the main Blender Org website. I tried to install from each one, but they all checked positive for a Trojan. The Trojan that was detected is ‘Trojan.Swizzor.c.lsxv.mg’. My antivirus is ‘Twister Antivirus’. I need to know if it is a false positive, or if this really is an infected file. Please verify your response.
Highly unlikely.
You can submit the .EXE through http://www.virustotal.com/ and get a better idea of what you have. You should only download Blender from Blender.org or Graphicall.org. (Yes there are others that are 100% safe, but these two are the prime locations for downloads.)
Twister is playing twist with you. I’ve used Blender on a different machines with at least five different antivirus programs and never ever had any problems with it. IMHO Graphical and blender.org are perfectly safe.
The so called ‘antivirus’ programs are so unreliable that they often act themselves as viri. E.g., I often see avalanches of complaints on DAZ3D because some antivirus program thinks that DAZ installers are viri or the binaries they install are so (I don’t remember a case of real infection of DAZ products and I have been a customer for theirs for years).
Remember that antivirus program developers might be in troubles if they failed to detect a virus, not if they cry wolf needlessly so… they cry wolf even for a hello_world.exe 
Thanks for your answers, I was anxious to use Blender, and I do think it was a false positive so thanks for reassuring me.