As far as I know you have to buy Apple Care WITH the computer. You might be able to get it now, although I've never heard of anyone doing it. Good luck!
You can buy the Apple Protection Plan anytime but it must be registered using the computer's serial number NO LATER THAN 1 year after the original purchase date of the computer....
So if you purchased in January 2009 then you have missed the boat unfortunately.....
APP only covers for 3 years (including the first 12 months) so even if you had purchased it in January 2010 it would only have covered you for the next 2 years....not the next 3 years.