How do I restore my Zip files?

I downloaded a 250 MB-odd file a few days back. When I attempt to open it I get an error report which says: "Cannot Open file: it does not appear to be a valid archive"

I remember having read a few days ago on the forum that there are softwares which help to restore Winzip files didnt get downloaded properly...

Anyone got any idea? Post back your replies ASAP....

Thanx