Steps:
1. Launch GW2 client.
2. Alt-tab to the desktop.
3. Click the GW2 taskbar icon twice quickly.
This seems to put the GW2 client in a conflicted state where it does not return focus to the client but it also prevents the desktop from refreshing so the PC is no longer usable and has to be hard-reset.
Found on:
Windows XP
Radeon HD 4000 series