Guild Wars "Unable To Install"

I just started getting this message when trying to start up Guild Wars:

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Unable To Install

The installer could not write Guild Wars to the specified directory. Please check that you have administrative access, that you have permission to install programs in the specified directory, and that there is sufficient space available on the drive.
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I am pretty sure that all these things are true (member of Administrator group, 80+ GB available disk space) but any ideas on what I should check?

I do have Norton Internet Security and have had to turn off the firewall to get it to connect at all, could Norton have found a new way to deny me?

Can you install anything else from CD? It may say that you have administrator priviledges, but you may not have full administrator rights. Try logging in as the administrator.

I am an administrator account on XP.

I just successfully installed something I'll probably never play (Star Trek Armada) just to make sure.

I would try deleting the directory GW is installed in, and try again. Might be a read-only thing or something.

I dunno. Rebooted my PC and now it works.

Thanks for trying to help Matsumi.