We investigated the activation of B in Si by a millisecond annealing process performed by infrared laser annealing. Reference samples were annealed in a lamp based rapid thermal annealing system at 200degC/s. We found that comparable diffusion length (~25 nm), the laser annealed samples show a Rs ~430 Omegasquare, corresponding to an active dose a factor two higher relative to lamp annealed samples. The laser annealing/activation process, lasting for just a few milliseconds, is likely characterized by the absence of B-interstitial clusters formation and precedes by far the regime known as "transient diffusion" with a duration of at least a few seconds at temperatures above 1000degC and which is driven by the formation/dissolution of large clusters involving B atoms.