Screen goes black after login following Sophos installation

Hi. I have been installing Sophos on various clients in our school, using the endpoint installer from Sophos Central. They have all been fine except for one PC. It's running Windows 7 and the installation seems to go fine, but when I log in to windows after the installation, the screen goes black. There is a mouse pointer but nothing else. I have to use Ctrl+Alt+Del to restart and use the F8 menu to choose Use last known good configuration, and then I'm able to log in. But this happens every time I reboot. I've tried uninstalling and reinstalling Sophos but I get the same result. Any ideas?