Blue error screen (BSOD or blue screen of death) are serious problems.
A driver or a part
This type of error indicate that a computer part, a driver
or any other critical Windows component is damaged.
There are no perfect solution of that type of problem, only numerous tricks that can help you to move towards a solution.
If the problem is new, you can press the F8 key on startup to try to use system restore to come back to a previous restoration point. This will cancel many changes recently made in the computer configuration and in the softwares but won't delete any document.
Find the software that makes the PC crash
If your comptuer still boots, you can try to identify the software that makes Windows crash so you can uninstal or reinstall it. A little software such as whocrashed can let you consult details concerning last crashes. You can deduct the role of suspected files with their name and location. For example, a file containing the word ATI is surely a part of a ATI graphic card drive installed on the PC.
Test the PC with Ubuntu
If the computer doesn't boot anymore, you can start Ubuntu
from its LiveCD to verify if the computer works correctly with Ubuntu. If that's the case, there are little chances that the problem is a real defect in the hardware.
Reinstall Windows, the ultimate solution