How do I stop files changing my units preferences when opening in illustrator

Question

When I open files in illustrator that have been sent to me they sometimes open with the measurement units changed from "mm" to "cm" or even "in", not a huge problem but when I have 10 files for a job that are all doing the same its an extra bit of messing around I could do without.
They can be any kind of file .ai .eps .fs .pdf etc..., when I go into my preference to change it back it shows that it is set to "mm" already?????. I can change it to "mm" again but I have to go Edit - preferences - Units - change them to read "cm" - OK, then repeat the process Edit - Preferences - Units - change back to read "mm" and that does it.

Answer 1

You are welcome, maxwells3.
When you have the time (and courage), you may have a go with some of the suggestions in the list below.
The following is a general list of things you may try when the issue is not in a specific file (you may have tried/done some of them already); 1) and 2) are the easy ones for temporary strangenesses, and 3) and 4) are specifically aimed at possibly corrupt preferences); 5) is a list in itself, and 6) is the last resort.
1) Close down Illy and open again;
2) Restart the computer (you may do that up to 3 times);
3) Close down Illy and press Ctrl+Alt+Shift/Cmd+Option+Shift during startup (easy but irreversible);
4) Move the folder (follow the link with that name) with Illy closed (more tedious but also more thorough and reversible);
5) Look through and try out the relevant among the Other options (follow the link with that name, Item 7) is a list of usual suspects among other applications that may disturb and confuse Illy, Item 15) applies to CC, CS6, and maybe CS5);
Even more seriously, you may:
6) Uninstall, run the Cleaner Tool (if you have CS3/CS4/CS5/CS6/CC), and reinstall.
http://www.adobe.com/support/contact/cscleanertool.html