Basically I want to end-up with kind of a fresh or newly purchased game wherein none of the missions are unlocked.
This may seem an odd question but I want to play WC3 again and having all those missions readily exposed or available ruins the experience for me.
It's easiest to create a new single player profile in the single-player menu above the campaign button. You can also delete the save-games for your chosen profile on the hard-drive. The save games with profiles should be located in the save folder in your game directory.
Installing it again would work but takes time and is absolutely not needed. Warcraft III and Warcraft III:TFT both being games that were able to be played offline, they have local campaign content and save. Which means they don't have cloud-like data such as Starcraft 2. With this in mind you can assume there is, somewhere in your installation, those saved games you want to get rid of. You can delete (or maybe just backup for latter) them. Find your Warcraft 3 profile, your in-game local name for your campain, and delete it! Also, you can just start another one. Being binded to directories, the game wont let you have the same name.
Have fun playing this super game again!
You just need to create new player profile.
You do so by going to "single player" menu and clicking on magnifying glass next to your current player name.
