The homepage of the distro they're for probably has an md5 checksum and/or a sha1 checksum. There are programs to calculate the checksums of files under Windows, just do a quick search. Compare the checksums of the files you've downloaded with the published ones. If they agree, your isos are correctly downloaded. If not, they aren't. By the way, there's no point in checking both md5sum and sha1sum, one should suffice.
As an aside, you might want to consider downloading by Bit Torrent in future, as this performs checking as it goes so there's no need to do checksums. It has the added advantage of taking a lot of load off the distro's servers.