Dimension c521 problems starting up

When I press the power button on my C521, the first screen comes up and the white bar moves to about halfway then the computer freezes, and the only way to solve is to power off wait for about 30 secs then power up. Any help re the above would be appreciated. Not sure if this has anything to do with the above but sometimes I get a message saying darwin.exe failed or something to that effect when shutting down the computer