What is the best way to calculate a checksum for a file that is on my machine?

Any MD5 will produce a good checksum to verify the file. Any of the files listed at the bottom of this page will work fine. http://en.wikipedia.org/wiki/Md5sum

The CertUtil is a pre-installed Windows utility, that can be used to generate hash checksums:

CertUtil -hashfile pathToFileToCheck [HashAlgorithm]

HashAlgorithm choices: MD2 MD4 MD5 SHA1 SHA256 SHA384 SHA512

So for example, the following generates an MD5 checksum for the file C:\TEMP\MyDataFile.img:

CertUtil -hashfile C:\TEMP\MyDataFile.img MD5

To get output similar to *Nix systems you can add some PS magic:

$(CertUtil -hashfile C:\TEMP\MyDataFile.img MD5)[1] -replace " ",""

I personally use Cygwin, which puts the entire smörgåsbord of Linux utilities at my fingertip --- there's md5sum and all the cryptographic digests supported by OpenSSL. Alternatively, you can also use a Windows distribution of OpenSSL (the "light" version is only a 1 MB installer).