I’ve got the same problem. GW kept crashing during the update, so I decided to reinstall the whole game. When I try to install the game the same error occurs. Disabling firewall, antivirus etc. didn’t help at all.
I also renamed Gw2Setup.exe and put it into separate folder. It created few files in that folder but crashes didn’t stop.
EDIT:
my specs:
Windows Vista Ultimate 32Bit SP2
AMD Radeon HD 7700 Series
Intel Core 2 Quad CPU Q9300 @ 2,5 GHz
4GB RAM