winMd5Sum is a free, open source MD5 sum checker that allows you to check and compare the md5 sums of files.
winMd5Sum Portable makes it easy to verify that the files you download are unaltered. Simply drag and drop a file to the window and it will calculate the Md5 sum in seconds. You can even copy and paste the published md5 sum into the compare box to quickly compare it.
An MD5 sum is a cryptographic hash used to verify the integrity of files. So, when you download a file, you can check what the publisher says the MD5 sum is and compare it to the MD5 sum of the file you have. More about MD5..
Also, there is the free application WinMD5Free.
Also, Microsoft has created the File Checksum Integrity Verifier command-prompt utility that computes and verifies cryptographic hash values of files. Download it here: http://support.microsoft.com/kb/841290.