First of all, all the files on our servers are checked, they are 100% correct. Now if you download a file and it will not unpack it could be that the file you downloaded is corrupted. In that case we STRONGLY recommend clearing the cache of your browser and to download the file again. Clearing the browser cache is done differently for every browser, search the internet and you will find easy steps for that.
If you are looking for a very simple compressions tool, check out 7Zip. It can handle any kind of file you will find and works very fast, very easy and does not come with any junk. Seriously good software.