I am runnig XindowsXP on standalone Dell PC. I tried to install an HP LaserJet 2300L printer. Wwnt through installation procedure using HP CD but received message asking for dot4usb.sys location, with a prompt to insert Windows XP installation disk. Did all that, and system came back saying it could not fine the file. Found another copy of file on my laptop and tried to use this via memory stick. Message came back saying software not signed by Microsoft. Previously this printer worked fine, using exactly same install proceure and same PC. Any ideas, anyone? Apart from throwing printer and PC out of the window?