I am going with Scorpei on your other topic (
http://gbatemp.net/index.php?showtopic=140960&hl= I assume).
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I will expand it however, a hard drive (conventional ones, solid state are different) works much like a record player crossed with a tape player in that it drags a mechanical needle across a rotating disc and uses magnetism to read the data (this arrangement by the way is why drives are so slow compared to other computer components).
If you dropped it while it was active as you say then the read head was likely slammed into the drive causing either problems in the head, a nice gouge out of the drive (magnetic materials are not generally known for their strength/toughness) or a crashed head.
You may have a chance with either a recovery company (which could take a while and not even work and you will have to pay through the nose for such things).
You could also put the discs into a new housing (preferably from the same drive type) however this is far from an end user technique (dust is not good here) and frankly if you have to ask you are likely not capable of it.
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Back on topic as it were installed games/files tend to have a record of such things contained within the registry, I am not going to suggest you edit it by hand (here is how you would do it if you were though
http://support.microsoft.com/kb/314481 ) but use something like spybot search and destroy on advanced mode (
http://www.safer-networking.org/en/home/index.html ) which has the option.
That might not work though as the installer will aim to detect another registry entry, what this I do not know and if you do not know what you are doing messing with the registry is not a good idea, general idea is either find them by hand or use another computer, dump the registry before install, install it and compare to see what is new (you can also run a change detector if you like and save yourself a bit of time).
