Sometimes when the operating system is loading it might encounter an error saying that a device could not be loaded or a service could not be started and would give a blue screen and not move any further.
The error might say "Windows failed to start because <name of missing dll> was not found" or it could be an IRQL error with junk characters.
The device errors are caused due to device conflicts or due to usage of incompatible drivers. And the Service errors are caused due to newly installed software's or due to missing files or corrupted Windows.
In most of the cases Windows should be able to boot to any of the advanced start up modes: safe mode, safe mode with command prompt etc.
Once in the Safe Mode, use the MSConfig utility to disable all the services except the critical ones (). The system should work fine even with non-critical services disabled, it should be noted that in most cases the system might have an anti-Virus software installed on it. The Anti-Virus SW service may need to be enabled.
Note: In case of Windows XP all the services can be disabled once you have a check mark on hide all Microsoft services under the services tab.
