Husband deceased July 2008. I have just turned 45 in March 2011, can I claim widow's pension?
Widow’s Pension was a weekly payment which you could claim if you were a woman without dependent children who was aged between 45 and 65 at the time of her husband’s death. The widow's pension ceased to exist, except for those already receiving it, in April 2001 when it was replaced with the Bereavement Allowance.
To be eligible for Bereavement Allowance, your had to be aged 45 or over when your husband died. So unfortunately this does not apply to you.
As an aside, widows below the state pension age with at least one dependent child may be able to claim widowed parent's allowance. For more details - Widowed Parent's Allowance.
I hope that helps and good luck