I selected "Windows 32 bit" from the menu; it installs 64-bit anyway. (The executable is named GW2-64.exe, not GW2.exe). The 64-bit version runs really badly; it's basically unplayable.
Where can I get the 32-bit client? That's what worked for me when I played last year. (I'm aware I'll need to use the -32 flag with it.)