OSX 7.5 All of a sudden I'm freezing and getting a dark square with the instruction in 5 different languages to emergency shutdown by pushing and holding the main button on my Power Mac tower and then restart. This is happening with great frequency..

That is a Kernel Panic.

What is a Kernel Panic?

https://support.apple.com/en-us/HT200553

and https://support.apple.com/en-us/TS3742

http://developer.apple.com/mac/library/technotes/tn2002/tn2063.html

More details here:

http://www.thexlab.com/faqs/kernelpanics.html

Useful articles on how to avoid and eliminate kernel panics here:

http://reviews.cnet.com/8301-13727_7-10344626-263.html?tag=mfiredir

http://www.macworld.com/article/2027201/how-to-troubleshoot-a-kernel-panic.html# tk.nl_mwhelp

As Klaus1 points out, these are kernel panics.

They could be caused by hardware or software.

From a hardware perspective:

Do you have 3rd party RAM? A common source of kernel panics.

In 2nd place Broken USB devices and/or a broken USB cable (can include keyboards, mice, external USB disks, etc...)

There are some Mac models that have hand graphics hardware problems.

From a software perspective, 3rd party kernel extensions, such as anti-virus packages.

I you could post a few of the kernel panics, that might help identify the problem (might not, but it can't hurt)

<http://support.apple.com/kb/ht2546>

You might also make another post with the EtreCheck output that will give 3rd party software information about your system.

<https://discussions.apple.com/docs/DOC-6174>