|re: Why would this problem occur out of no where?|
Well, as I'm sure you know, it didn't occur "out of nowhere".
Some setting changed on your machine and Excel thinks it needs something installed before it can open. Windows (and most applications) make changes to the registry and to their own configuration files all the time. All it takes is a glitch here or a hiccup there and something doesn't get set/saved correctly.
How often have we all had files that looked perfectly fine when we saved them, only to find that they are corrupted the next time we try to open them? Something didn't save correctly and the file is toast.
The utility I suggested cleans up the Windows Installer, which is notorious for leaving things hanging that can cause problems later. I don't know if it will solve your problem, but as it says at the website:
Windows Installer CleanUp Utility does not perform the following functions:
- Remove Windows Installer
- Remove files of any programs that are installed by Windows Installer, such as Microsoft Office 2003
Therefore, IMO, running it won't hurt anything and it may just clean up whatever is causing Excel to think it needs to install something.