Alex headed the Blues in front on 16 minutes, only for Scott Parker to smash a 25-yard effort past rookie goalkeeper Ross Turnbull and level against his former club.
However, Chelsea took full advantage of Manchester United not playing until tomorrow when Didier Drogba (55) headed in Florent Malouda's cross and the French winger capped a superb display when he netted a third with 13 minutes left.
Former Hammers man Frank Lampard hit the post, before Ivory Coast striker Drogba scored his second in stoppage time as Chelsea went top in style.
At the other end of the table, Wolves put some distance between themselves and the bottom three with a 2-1 win at struggling Burnley.
Matt Jarvis (26) fired the visitors ahead after latching onto a poor back-pass from Tyrone Mears.
Clarets striker David Nugent saw his close-range effort bounce up onto the top of the crossbar, and Burnley's luck was out again when Clarke Carlisle deflected Adlene Guedioura's shot into his own net at the start of the second half.
Steven Thompson (73) set up a grandstand finish with a goal for the hosts, but Wolves held on to secure a much-needed victory.
In the early kick-off, Tottenham maintained their hopes of a top-four finish as they beat Blackburn 3-1 at White Hart Lane.
Roman Pavlyuchenko netted a second-half brace to take his tally to eight goal in six games, after Jermain Defoe (45) had put the hosts ahead just before the interval with his 17th Premier League goal of the season.
Chris Samba (80) pulled a goal back, but the in-form Russian had the final say to wrap up the points for Harry Redknapp's side.
Bolton continued their resurgence under Owen Coyle as Wigan were thumped 4-0 at the Reebok Stadium.
Swede Johan Elmander (10) put the Trotters ahead, but Jussi Jaaskelainen had to produce a fine save from Charles N'Zogbia to keep the hosts in front.
Kevin Davies converted a penalty at the start of the second half after Gary Caldwell had fouled Tamir Cohen.
Fabrice Muamba (53) then netted a fine individual effort to get his first Trotters goal, with Matthew Taylor (69) putting the seal on a third win in four.
Birmingham fought back from 2-0 down to draw 2-2 against in-form Everton at St Andrews.
Victor Anichebe (19) and Ayegbeni Yakubu (22) put the visitors - who have lost just twice in the League since the end of November - well in charge.
Alex McLeish's men, however, have an impressive home record, and got themselves back in the match through Cameron Jerome (26) before Craig Gardner (52) secured a share of the spoils.
Stoke and Aston Villa played out a goalless draw at the Britannia Stadium, where chances were few and far between.
Stiliyan Petrov had a shot on target in the 27th minute, but goalkeeper Thomas Sorensen got down low to his right to make the save, while England midfielder James Milner was just off target with a late free-kick.