Zubrus put Washington ahead 2-1 in the first period with two goals in less than 2½ minutes - both with help from reigning rookie of the year Ovechkin - and Lawrence Nycholat and Semin scored 56 seconds apart in the second period.
That was it for Canadiens goalie Cristobal Huet, who was yanked 8:48 into the second period and cracked his stick across his knee after leaving the ice. Huet came into the game leading the NHL with a .930 save percentage, but he allowed four goals on 19 shots before giving way to David Aebischer.
Semin flipped the puck over Aebischer's left shoulder midway through the final period for his 19th goal to close the scoring.
Washington goalie Olie Kolzig allowed Craig Rivet's power-play goal in the first period, and that was it, finishing with 27 saves. Kolzig flipped to his back while making a fantastic reflex stop on Michael Ryder's putback late in the second.
Ovechkin's season-worst goal drought reached five games. Still, he reached the season's midway mark with 54 points, three more than a year ago. As a whole, the Capitals have shown even more improvement, reaching the halfway mark at .500 (17-17-7). A year ago, they were 10 games under .500 through 41 games.
The Capitals had won only three of their 21 previous games this season in which the opponent scored first, and Montreal went ahead on Rivet's fifth goal at the 9:34 mark, off assists from Alex Kovalev and Guillaume Latendresse.
But Washington, which lost to visiting Montreal 4-1 on Dec. 27, tied Thursday's game at the 15:21 mark of the first period on a power play. Ovechkin took a shot that sent Huet sprawling, and Zubrus put in the rebound, then dropped to his knees and raised both arms.
Zubrus' celebration was more muted at 17:43 when he put the hosts ahead 2-1 with a close-range shot while Ovechkin screened Huet. That gave Zubrus 16 goals this season and 200 points with the Capitals, a team he joined in a trade from Montreal during 2000-01.
The Canadiens thought they tied it 7:18 into the second period during a power play, but the apparent goal was waved off because Saku Koivu was whistled for goaltender interference.
Washington padded its lead during the ensuing 4-on-4, when Ovechkin found Nycholat all alone near the left post for a tap-in at the 7:52 mark. As that goal was being announced, the Capitals went up 4-1 when Boyd Gordon's shot was saved by Huet, but the rebound went right to Semin.
Notes: When Zubrus scored a career-high 23 goals last season, he didn't get No. 16 until March 22, in Washington's 68th game. This was his fourth multiple-goal game of 2006-07, twice as many as in all of 2005-06. ... Ovechkin was sent off for a four-minute penalty for high-sticking late in the first period and Washington managed to kill it off against Montreal's NHL-best power play. ... Koivu went scoreless and still needs one point for 500 in the NHL.