help with downloading and extacting

When i download a game i downloaded MK4 and MKM:Sub-Zero i go extact them and i get error CRC file failed file is corrupted and below it says Mortal Kombat 4 [U] [SLUS-00605].rar Unexpected end of Archive
dose any know why it dose this?

Unstable internet connections can cause downloads to stop. Then once stopped, your browser assumes the file is complete and says it downloaded the whole thing, when it might not have.Use a download manager like http://www.flashget.com/ to be able to resume stopped downloads and avoid corrupted files.

dosent firefox come with one of those?

Firefox has a built in download manager, but it is not as good as a real one.Flashget integrates very well into Firefox. Download the "FlashGot" addon for firefox :)

Ok then thanks for the info.