Cannot delete/view/modify specific filetype

Using 32-bit Vista Home Premium for several months without issue.

As of last night I've noticed being unable to delete/view/modify any files
of one specific filetype universally across my system. The files are
Photoshop project files (.PSD) and can be deleted only in safe-mode. I can
understand if this would be isolated to one or even several files that were
corrupted, but not every previously working file in any location.

To be specific, upon trying to open, delete, rename, move, cut, copy, view
properties of, or try to access a file of this type (.psd), whichever program
is trying to complete the action will hang indefinitely. As example, trying
to delete doesn't give an error message, the system just displays the
'Calculating time remaining' which can be cancelled; or trying to rename will
cause Explorer to stop responding; or using HijackThis to 'delete on reboot'
will lock up the program when selecting the file in question. Even a command
prompt running with admin privileges will hang on "del filename.psd". And so
far as reinstalling Photoshop or somesuch solution, why would Windows need to
consult with a file's associated program before trying to enforce a
system-only action like deletion?

Can anyone shed some light on specifically what could cause this to happen?
So much as system restore might correct the problem, I'd like to know what's
behind it.