stolen from FW:
If you haven't already, you should check to make sure your checksums are correct.
You can also use DigestIT 2004. It adds verify and calculate of md5 and sha1 hashes to your right click context menu. It is very simple.
I use md5deep. All you do is download it and put the exe's in your C:\Windows\System32 directory.
You run it in command prompt (DOS) by going to the start menu, selecting "Run" and entering cmd.
In command prompt, go to the directory you downloaded Vista to and type "md5deep vista_5744.16384.061003-1945_x64fre_client-lrmcxfre_en_dvd.iso" (for the 64-bit version). Then press enter. It will appear to hang, but that just means it's processing.
You can also check the sha1sum with it, by doing "sha1deep vista_5744.16384.061003-1945_x64fre_client-lrmcxfre_en_dvd.iso" You can only do one at a time (well I wouldn't recommend trying to do more... it's slow enough with one).
If you checked it right you should get a response like:
"23b213d7342b2e957b3db1d98c0ee3e6 E:\vista_5744.16384.061003-1945_x64fre_client-lrmcxfre_en_dvd.iso" E: just happened to be the hardrive I downloaded to. It should be where you downloaded the iso to. Make sure the number before the file name matches the checksum(signature) listed below for that version and type of checksum. If it's not, you have a corrupt file and need to re-download.
HINT: You can start typing vista then press tab and the rest of the file name should pop up so you don't have to type the whole thing. If you downloaded both versions you can press tab again to switch between them so all you should need to do is type vista and tab a couple times, most to get the whole file name.
If you haven't checked the checksum you are probably still fine, but occasionally there have been problems with the MS servers and since they provide them you may as well for peace of mind.
Signatures of the ISO files (checksums):
32-Bit
MD5 hash: 83cee5f1642d094670147c5350c57762
SHA1 hash: 067CA4F94D001DE6276F08F76EF0780592B64BFD
64-Bit
MD5 hash: 23b213d7342b2e957b3db1d98c0ee3e6
SHA1 hash: 5D8C4BCEE51F6912801800E534F6DB5A22F654AE