As you install, delete, and obtain more files on your computer, the information on these files is kept stored. This information is randomly stored, thus separating each bit of information. When this happens, the file is called a "fragmented" file. By defragging your computer, you are taking the fragmented files and organizing them together. When a file is fragmented, it could take your computer 1 second to an entire day looking for the file.
As said by Toasteddude write down or take a picture of the BSOD and post it up here so we can help you further. CPU overload is often the case with the BSOD, because the BSOD was originally created to keep a corrupted file or partition on a computer from causing further damage. Definitely post your system specs, and the BSOD up here asap so we can help your problem ;]