Dell Inspiron 1545 Battery: Windoze 7 tells me my freestyle needs to be replaced when it doesnt.?

i hold a 1 year old dell inspiron 1545 laptop. Windows 7 keeps telling me to consider replacing my battery. I did a test by disabling the ACPI (battery reporter) and my battery lasted 2 hours. when i re-enabled and tested unplugged it lasted 20 minutes. However do i fix this problem with Windoze?