We can't help you if you don't provide more information. Where does it stop loading? Any error message? BSOD? Does it power on at all? Can you hear fan spinning? Etc...
If it's a Windows problem you can boot in "safe mode" and use "system restore" from there.
If that doesn't work then you can use your recovery cd (or hidden recovery partition) and reinstall Windows, but that would be destructive in the sense you will loose any data that was on the laptop.