laptop startup error

This is my daughters laptop. When I switch it on, it tells me that Windows has not been shut down properly
i) if I take the normal boot up option (not safe mode), CHKDSK is automatically initiated.
ii) After this has run through, the normal user icons appear. I click on user icon, enter password, screen blanks then returns to user icon (logon) screen.
iii)On trying to logon again, screen turns blank and stays blank (although cursor arrow is on screen and moveable with mouse pad).
Nothing else happens even if left for an hours or more.
ADDITIONAL: when this first happened, logged in ok then the following error message appeared, "The instruction at 0x74eab55a referenced memory at 0.00000000. The memory could not be written."
SAFE MODE: I can login ok but then have the folloeing error message displayed, "This application has failed to start because MSVCR80.dll was not found. Reinstalling the application may fix this problem."
Help please!!