Dell E1505 Battery Is Dying On Me....

I have a Dell Inspiron E1505. I've had it for about two years. I never travel and I never leave it off the charger for long. Now I'm getting an error message telling me that "my battery is able to charge normally but it's reaching the end of its usable life. I'm not sure I understand why the battery has "usable life," especially when the computer is only about 2 years old.
I'm also curious about how it is that Dell can put out laptops with batteries that have to constantly be replaced. If the computer is treated appropriately, then there shouldn't be any issues, right? I mean, you can charge and recharge regular batteries - what is so different with this situation?

Could someone please advise me on the best route to take with this problem? I'm a broke college student who needs this laptop for both work and homework.

I'm sure I purchased a warrenty with this computer, but I'm not sure if this is covered.

I have no idea what to do :(