Ok
First
turn over your computer, underneath there is a white sticker with the name "service tag" on it, and then some numbers and letters
put it into this website: http://support.dell.com/support/topics/global.aspx/support/my_systems_info/en/details
and see if your extended warranty was registered by the people you bought it from
to my best knowledge, extended warranty needs to be registered within a week of purchasing the computer, or there is no use for it.
If the warranty was not registered, take the receipt of you buying the extension to the place you bought it, and demand they fix your computer,or your money back for the warranty because they didnt register it.
If they did register it,contract the representitive you spoke with about your computer who told you they did not want to honor your warranty.
If the problen in your computer caused by hardware failure they shouldnt deny you service.
If the problem is software failure they may deny service, because therey is no warranty on software.
When you bought your computer, did it include an operating system?
If so, there should be an OEM sticker at the bottom of the laptop (turn it over)
If the sticker is there, ask the computer place where you bought the computer for an OEM matching software disk to install your OS(assuming the OS is windows)
Why did they replace your harddrive? was it defected?
Ask them to give you output of every single thing done on your computer
i.e replacement of harddisk, memory, motherboard what not
And in the future, buy a toshiba, they have much much much better service ;)