I have a Dell XPS 4435MT desktop. It randomly freezes – and it happens shortly after startup, mostly less than 10 minutes.
When it freezes, mouse and keyboard have no power anymore. The USB ports on the front and back do not have power until reboot. Upon reboot it will normally work until a normal shut down.
It's really a hit-and-miss situation after start up. It seems like Article 54401 on Dell's support site covers this. My chipset is different at Intel ICH10 family though.