Eugene's observations

It?s almost inevitable that at some point circumstances will conspire to cause Windows to fail in some way; this may take months or years depending on how the computer is treated and a large helping of luck. 

When it freezes the only option sometimes is to turn the power off abruptly.  However when this is done, any files that Windows had open in volatile memory will not be flushed out to disk and will therefore usually become corrupted.  Whether the particular files thus affected are significant enough to prevent Windows starting next time or merely cause it to misbehave is down to luck.  Each time this happens the problem is exacerbated until eventually a catastrophic failure (where Windows is unable to start) is bound to happen.

So initially may be reasonable to ignore the cause but at some point it would be sensible to attempt to resolve the problem.  This can be done in one of three ways.

1. The easiest and least painful would be to allow Windows to repair itself as far as it is able.  The procedure is explained in my blog entry Disk & Windows repairs 
   
   
2. If the first step does not resolve the issue reinstalling Windows on top of itself is an option that takes a little longer but stands a better chance of resolving the issue.  This ?overlay? or ?repair? installation is not a guaranteed fix as any non-Windows programs that have been installed will not be touched, so if one or more of them are conflicting with Windows attributes the problem may remain.  However it is often favoured over the final option as at the end of the procedure the computer may work as it did before the problem without any further action.  This is only possible on versions of Windows after XP if the computer can get into Windows.
    
3. The final option is a certain way to resolve all software problems with Windows: wipe the disk clean by reformatting it then install Windows from scratch.  Not only is this option the most time consuming but it requires all programs and device drivers to be reinstalled, and that means locating the installation sources and any activation keys they require.  Also, and photos, documents, music, e-mails, browser favourites and other additions to the computer will have to be copied onto a backup device before the operation then restored after.