Not using the current .dat file

It would appear that my Guild Wars, which I recently re-installed to the D drive from the C drive and transferred the .dat along with, is attempting to make a new .dat file somewhere else. I've gathered this from the fact I have to download something pretty much everytime I have to enter an area, a huge amount from the first area I entered, definitly.
I don't know where this somewhere would be, how would I know, and how do I make it use the old one?

Is the path in your guildwars.exe still directing towards the C disk? That'd explain it.

Not that I can see :/

By "re-install" do you simply mean "copy". If you simply copied it, all the paths in the registry would still point to C:.

The best way to do this would be to copy the entire Guild Wars folder to some temporary location (just in case, so it doesn't get deleted) and then to "un-install" GW. That would (should) remove all the paths from the Registry.
You can then copy Guild Wars back to D:\Program Files\ and simply run (double click on) GW.exe to get the paths to point to D: