Trouble installing guild wars on a computer that came with Windows 7

During the installation guild wars eye of north, it would ask me where to save guildwars.exe, it came up by default with C:\computer\Program Files(x86)\Guild Wars. I clicked ok and the window disappeared. So I clicked the guildwars icon on the start menu to run it and it comes up with a message saying

"Unable to open Archive File

Guild Wars is unable to access its archive file C:\Program Files(x86)\Guild Wars\GW.dat. Please verify that this file is accessible and writeable, and that no other program is using it."

I have no idea what that means and I have very little computer knowledge so if anyone could offer a fix that would be great. Thanks in advance.

Next time, try right clicking on the shortcut and choose "Run as administrator". That may fix any permission issues. (You should only need to do that once).