Well, I know it sucks but I guess I'd wipe all the temporary files and try to re-download. Don't know what else to suggest, sorry. May also be worth trying deleting the registry keys. Trying to install to the default path might also be worth a shot; you never know.

To close this one off... i found something that fixed it.
I don't understand it but whatever...

I have a OCZ Vertex 3 SATA3 SSD for my Windows7 installation (so not the disk i was installing TW2 on, that drive is a Vertex2 SATA2 SSD) and after updating the firmware of the Vertex3 i was able to play! So i'll be off hunting now, finally! :-)

EDIT: hmm, can't mark my own post as the solution...