Hi!
I have a 3ys old notebook Acer TravelMate 212TXV. I changed the 10GB HDD to 40GB Seagte and it couldn't be detected by the BIOS. What shoudl I do? I am not much of an expert in IT. Thank you.

Chances are the reason why it was not detected was because it did not have an Operating System on it. I would take both HDs to a computer shop and have them transfer the information over from the old HD to the new one. It can easily be done if they were desktop HDs, but notebook ones require additional hardware and different techniques. To give you a round-about price range, I charge $25 for desktop ones. They should not be more then $5-10 difference.

Edited: To show them that you have Windows, I would take the entire notebook with you. This way, they can see that you have a legit copy. Some may ask for it, and others may not.