Windows Xp Back screen on start, No task manager access, mouse works fine

hey i am having the above problem
Windows Xp Back screen on start, No task manager access, mouse works fine, when i press the sleep button, the computer says, please wait while xp goes on standby this signifies that xp is fine, when i press ctrl alt del, nothing happens, when i press ctrl shift esc, nothing happens, ctrl esc, nothing happens. i do not have xp cd as it cale pre installed with the pc. i have tried everything i can think of. i cannot access anything on the computer sp i cannot run virus scans.
please help urgently?
thanz