What is the $Wb\bar{b}, Zb\bar{b}$ or $t\bar{t}b\bar{b}$ irreducible background to the light Higgs boson searches at LHC?

The W b anti-b, Z b anti-b and t anti-t b anti-b production at LHC are irreducible backgrounds for possible observability of the Standard Model (SM) and Minimal Supersymmetric Standard Model (MSSM) light Higgs boson respectively in the associated WH, ZH and t anti-t H production followed by the H -> b anti-b decay. We present a comparison of the background estimates as obtained with (a) complete massive matrix element implemented in AcerMC Monte Carlo generator and (b) PYTHIA implementation of the inclusive W, Z, and t anti-t production, followed by the parton showering mechanism. Both approaches lead to a production of the final state of interest, but differ in the approximations used. We concentrate on the comparison of these approaches for the background to the light Higgs boson searches at LHC.