I have never been overly cautious about checking the integrity of files that I download, unless the file is a critical file to the operating system, like drivers and such. In years, I have not seen an error and yet I still check. But, today! I downloaded the Smart Array controller driver for my ESXi 5.1 server. As usual I checked the file and was surprised to see that the SUMs did NOT match. So, I downloaded it again and the SUM Matched the SUM on the other file and not the website.
Being ultra cautious I contacted HP to have them verify that the SUM listed on the Page was correct for the file. I do not want to have a driver on my server that will cause a PSOD (Purple Screen of Death, like a BSOD (Blue Screen of Death), only a prettier color). To my surprise, the technician downloaded the file and it was the same as on the website.
I left my office and went to a coffee shop to download the file and it was correct. So, my recommendation to you is if you see an MD5Sum on a website, you check it before you run it.
How do you do that you ask? In linux you simply use the command sum or md5sum and the name of the file. It is usually installed on the operating system.
In windows, you need to get a tool to do this simple task for you. There are many for sale windows tools to check the sum.  These are not required (unless you are afraid of the command line for some reason). In my opinion the best choice is to just goto Microsoft’s website and download Microsoft’s MD5Sum tool, called “Microsoft (R) File Checksum Integrity Verifier“.
This comes as a self extracting zip file with two files in it. The readme.txt will give you all the direction you need to verify that you have actually downloaded the file you thought you were getting.
The usage is simple
C:\>fciv.exe c:\mydir\myfile.dll
//
// File Checksum Integrity Verifier version 2.05.
//
f7668e5f069ffc621a2d9ddd4aa60ec0Â myfile.dll
I like to pipe that through to a find so that I do not have to check that against the sum manually.
C:\>fciv.exe c:\mydir\myfile.dll | find “f7668e5f069ffc621a2d9ddd4aa60ec0”
f7668e5f069ffc621a2d9ddd4aa60ec0 scsi-hpsa-5.0.0-28oem.500.0.0.472560.x86_64.vib
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