You will have to copy the files over from the OLD HDD to the NEW HDD.
The file should end in *.ACC or *.M4A and if you have Videos they will be *.M4V.
You can find them in: C:\Documents and Settings\YOUR USER NAME\My Documents\My Music\iTunes [In XP/iTunes default location].
However you might have to take ownership of the files/folders: click here [Although it's for XP, it's more or less the same for Vista].
In Vista just import them into iTunes and you should be able to play them. You will probably have to authorise them first but they then should play. In iTunes in your account options you can Deauthorise all computers if you want and start again.