This kind of thing can happen when a game is installed and remaps the behaviour of the keyboard or mouse, but fails and when rebooted, messes up Windows.
Dell generally has a set of keys defined on bootup that lets you into its diagnostics. It varies from machine to machine. Sometimes it is ALT-F9 or some such. Your manual should tell you. Hitting these will bring up its diagnostics. Run them. Sometimes, it will fix the problem, more often, it will give you an error code. Write that down. Then, call Dell and give them the code you found. It is better to use Dell's tech support rather than hack around and make it worse, particularly since you may have to send the machine to Dell.