Baros pounced from the spot in the 80th minute after Villa old boy Steve Watson was adjudged by referee Rob Styles to have handled a fee-kick from Gareth Barry.
The decision left Albion fuming and Watson plummeting from hero to zero in the space of four minutes. Watson had drilled Albion back on to level terms in the 76th minute with his first goal for the club since arriving from Everton last summer to cancel out Villa's opener from Steven Davis.
Albion were inches away from making the perfect start after a neat three-man move in the fifth minute.
Kanu released Diomansy Kamara down the right wing with a neat pass. Kamara's cross then flew across the face of the Villa goal to Jonathan Greening whose first-time shot with his outstretched leg flashed just wide of the far post.
In a typically frenetic opening to a local derby, clear cut chances were few and far between early on but Villa should really have broken the deadlock in their first meaningful attack in the 27th minute.
Baros made a powerful run from the halfway line and flicked the ball into the path of Davis who was unmarked in the right-hand side of the area.
Davis floated a pass into the path of Luke Moore but his first-time volley from eight yards crashed off the post.
That near miss really sparked Villa into life and Albion had to ride their luck three times in the space of two minutes to remain on level terms.
The lively James Milner saw his 20-yard shot in the 31st minute deflected wide and then the on-loan Newcastle United winger almost scored direct from the resulting corner when only a header from the well placed Paul Robinson prevented the ball going in.
Goalkeeper Tomasz Kuszczak then came to Albion's rescue by tipping a shot from Gavin McCann over the bar.
Albion continued to ride their luck on the stroke of half-time when another Milner corner caused chaos as Olof Mellberg nodded the ball down into the path of Mark Delaney who somehow managed to lift the ball over the bar from two yards.
But their luck ran out two minutes into the second half when Villa made a deserved breakthrough.
Once again, it as Milner who caused havoc as he raced to the goal-line and delivered an inch perfect cross to the far post for Davis.
Davis may be one of the more diminutive players in the Premiership, but he was unmarked and had was able to calmly head past Kuszczak for his sixth goal of the season.
Having dominated before they scored, Villa would have been expected to try and press home their advantage.
But all the goal did was to fire up Albion who gave Villa a severe examination as they went in search of an equaliser.
Villa goalkeeper Stuart Taylor, who was making only his second start after his summer move from Arsenal, twice came to his side's rescue - tipping a Greening cross over the bar and then pushing away Nathan Ellington's header.
Albion also had strong claims for a penalty turned down when Aaron Hughes appeared to handle the ball under pressure from Kanu, only for Mr Styles to wave away their appeals.
But their pressure was justly rewarded 14 minutes from time when Watson pounced.
The goal was of Villa's own making as Mellberg and Hughes both failed to clear a Greening free-kick and Watson lashed home the loose ball through the legs of Moore.
At this stage, Albion looked the team most likely to go on and win the game only to be undone by Styles' controversial decision to award Villa a penalty.
Watson looked hard done by to have been adjudged to have handled Barry's free-kick, but Styles had no hesitation in pointing to the spot and Baros took full advantage to roll home his sixth goal of the season.