RE: Laptop is not booting. It starts booting but restarts before it gets boot properly.
First thing to do is run an extended (NOT just 10 minute) diagnostic on the hard drive. If you need further assistance, post the results of that (numeric error codes) along with the full model information - i.e., Inspiron N5110, XPS L501x, etc.).
Press F12 a few times immediately after powering on to get into the diagnostics.