Rabobank's Boom finished two seconds behind stage winner Alessaandro Ballan at the end of the race's queen stage, a 208km route from Maldegem to Geraardsbergen.
And with his contenders for the overall victory finished down the order, Boom won by 26 seconds over Sylvain Chavanel (Omega Pharma - Quick-Step), with Canada's Svein Tuft (Orica) a further 14 seconds back in third place.
Alberto Contador, taking part in his first race since returning from a doping ban, finished fifth overall after ending the seventh stage in a group of six riders who were just six seconds behind Ballan.
