How are adults generally portrayed in Harry Potter and the Sorcerer's Stone?

This is actually an interesting question to consider, especially when looked in the context of the larger Harry Potter Series. Personally, I'd suggest that the adults in Harry Potter and the Philosopher's Stone are largely painted as archetypes, and perhaps one can even call them incidental to the larger story it follows (not including Quirrell and Snape, who are very much critical, as main villain and red herring respectively). Nonetheless, I'd suggest that, as characters, they're primarily defined in terms of how they impact the lives and struggles of the children, whose perspective the story follows. There are positive authority figures (as was already pointed out) such as Dumbledore and McGonagall, and there are negative authority figures such as Filch, Snape, Quirrell (and Voldemort), and the Dursleys. But as multidimensional and complicated characters, none of the negative authority figures are yet given much of a spotlight.
What's interesting is how this changes and evolves as the series continues across the seven books. In Book One, for example, Snape is characterized largely as a bully, a cruel schoolteacher, and a recurring obstacle Harry must overcome (though at the end of the book, we found out how he'd been protecting Harry too). In later books, we'll find out more about his own deeply complicated past, with Lilly, the Marauders, the Death Eaters, and his own role as double agent in the war against Voldemort. Similarly, there's Dumbledore, who is initially defined as a mentor and protector, though by the last book, we start to see the severe trauma and fragility which lies beneath the image—the tragedy with his sister, his history with Grindelwald, his own distrust of his own ambitions. As Harry grows up, we start to see with greater clarity the tensions and turmoils of the adult world he'll be joining.

The adults in Harry Potter and the Sorcerer's Stone (the first Harry Potter novel) cluster in two opposite categories: the bad authority figures and the good authority figures. 
Bad authority figures include Harry's muggle guardians, Petunia and Vernon Dursley. Petunia is the sister of Harry's mother, Lily. Both Petunia and Vernon have taken Harry in unwillingly. They treat him abusively and favor their own spoiled son, Dudley. They force Harry to sleep in a closet under the stairs. They let Dudley punch and bully Harry. Both parents fear doing anything in the least bit nonconformist. They worry that Harry will turn out like his wizarding mother—which, of course, he does.
When Harry gets to Hogwarts, his life improves immensely, but he still runs into bad authority figures, such as Professor Severus Snape, who dislikes and bullies him.
At Hogwarts, however, Harry finally gets under the tutelage and protection of some wise and kind (if at times gruff and stern) authority figures, such as the school headmaster, Dumbledore, and Professor Minerva McGonagall— both of whom have Harry's best interests at heart. The giant, Hagrid, the school's gamekeeper, also becomes a friend and protector of Harry.