If a download is corrupt, use the Verify Integrity of Game Files tool in Steam settings rather than re-downloading from unverified sources.
Third-party sites often claim to compress the game into sizes as small as 500 MB or 1 GB. This typically leads to several issues: Mafia II Highly Compressed PC Game
Pentium D 3GHz / AMD Athlon 64 X2 3600+, 1.5 GB RAM, and NVIDIA GeForce 8600 / ATI HD2600 Pro. If a download is corrupt, use the Verify