Missing information:
01. MacOS X version (example, 10.4.4).
02. Mac model (example, PowerMac G4 - APG Graphics, etc.).
03. Capacity of hard disk drive.
04. Amount of remaining hard disk drive space.
05. Steps used to attempt to back up disk.
06. The web site url where the Photoshop demo was downloaded from.
07. The size of the downloaded Photoshop demo file, on your Mac.
To obtain the required information of 01 through 04. above ...
Select the 'Apple, About This Mac' menu item. A 'About This Mac' window will appear. Answer 01. with the 'Version:' number listed (under 'Mac OS X').
01. Click on the 'About This Mac' window's 'More Info...' button. 'System Profiler' will launch, displaying a window.
02. With 'Hardware' selected in the left side list, look under 'Hardware Overview:' in the right side list. Answer 02. with the 'CPU Type' and 'Sales Order Number'.
03. Quit 'System Profiler'.
04. On the 'Desktop' locate and click once on the upper right most hard disk drive icon, then select 'Finder's 'File, Get Info' menu item (or do 'Command I'). Under 'General:' locate 'Capacity:' (answer to 03.) and 'Available:' (answer to 04.).
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No, you will not have to reinstall MacOS X - from a defective operating system perspective. If you install a larger hard disk drive (Mac model dependent), then yes - you will have to install MacOS X onto it.