O/S does not work

Question

I have dell laptop c840. it had xp pro sp2 was working fine then i installed sp pro sp3 worked ok . after few days I try to download online songs then it did not start windowns anymore. It just show xp pro screen and then flick blue screen for a second and goes to black screen and shows that i can turn on in safe mode or most recently used windows. Did not run window. any helpprinters