I have, quite a number times, experienced this bluescreen error and I discovered that it is ofttimes caused by missing system files and error in the computer's registry.
Missing System Files: This usually occurs when you (unknowingly) tamper with the windows/system32 folder files, or occassionally by some worms or virus infection. Oftentimes, before the computer shows the bluescreen, you will notice it complains of some files not found (usually .dll files)
Registry Errors: Even if you don't do much with your computer system other than to type documents, play music and read pdf files, these registry errors still occur. Whenever you install an application, no matter how small, even Google toolbar, it always makes some changes to your registry and add new registry keys. Sometimes these changes become ill-organized and cause issues for the computer. Worst of all, if some keys get missing or refers to system files that are non-existent, the computer often shows system errors. On rare occasions (often triggered by unclean shutdown and uninstallation of some software), when the errors are much the computer may not be able to start-up leading most times to bluescreen
For the first case, make sure no one tampers with your system files and use a password-ed Administrator account, not forgetting the default Administrator account (if you use XP), and get a good antivirus to protect you from virus attacks.
For the second case, always avoid unclean shutdown, be careful of what applications you install, 'cos when you uninstall an application there might be registry errors. So install only applications you trust and you will need. Also avoid hibernating all the times as it does not have the registry cleansing effect of a proper shutdown. Lastly and most importantly, use a registry repair tool or application to put the registry in proper shape occasionally, especially after series of installations and uninstallations.
In the first case, try to enter through safe mode and do system restore to an earlier date. If safe mode is not starting up also, you can do Advance repair/Recovery Console for XP using the XP bootable CD or try system restore using the Vista bootable DVD for Vista.
In the second case, you will most likely be able to enter through safe mode, just install any registry repair tool (be aware that some would not install in safe mode) and run them to repair the registry. I have actually done this before and the computer worked fine thereafter.
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