This study will assess the immunogenicity of GlaxoSmithKline (GSK) Biologicals' (formerly SmithKline Beecham Biologicals') DTPa-HBV-IPV/Hib (Infanrix hexa™) vaccine compared to the separate administration of DTPa-HBV-IPV (Infanrix™ penta) and Hib (Hiberix™) vaccines administered at 3 and 5 months of age.