H1-B's social security tax

My H1 started in Jan. 2003. I just noticed that I wasn't charged social security/FICA tax for the entire 2003. I know that I am supposed to pay for that tax. I think that my company's Payroll Dept. forgot to change it when my H1 started. So, what should I do? Tell IRS immediately or contact my company's Payroll Dept.? Is this a serious problem?

我见 和 假博士, and other experts, please advice. Thanks a million.