Your hard drive is failing. Make sure to backup what files you need and replace the hard drive. My recommendation is the Western Digital Black Series. You'll need a 3.5" hard drive.
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What's happening is files pertaining to your user profile are corrupted and can't load it properly. Your personal files are there, but it keeps loading a default windows profile since it can't load your profile. If you're not seeing your files, you can find them by navigating from Local Disk C: under My Computer, Users, than your profile.
It looks like the Seasonic PSU should fit just fine.