Anywhere where the game is offered for free should be avoided. The game is being sold as a digital download and has also had two all-new expansion packs released for it in the last few years.
I would get the Titan Quest Anniversary Edition from Steam, which you can find
here. It's only five dollars right now (great sale!) and works great on new systems. It also includes the original expansion Immortal Throne.
This is a remastered version with many improvements and fixes. You can read about it on the Steam page.
There have also been two additional expansions released for the game in the last few years - you can get the Anniversary Edition plus the two new expansions in the Titan Quest Bundle, which is also offered on the page that I linked to above.
GOG also has the game but it's not on sale at this time so everything Titan Quest is much cheaper on Steam right now.