There was no zero tax rate in 2008, and your long term capital gain should be taxed at 5%.
If you have $15,000 if taxable income in 2008 and you are 65+ and single
-- your standard deduction would be $5450
-- additional senior deduction $1350
-- personal exemption for one person $3500
Total exemptions - $10,300
Your taxable income will be $4700
Estimated tax liability assuming long term capital gain - $235
Plus, you will have penalties for late filing and late payment.
Let me know if you need any help.