When changing the priority of a process Windows 7 warns:
Changing the priority of certain processes could cause system instability.
Should I care? what does it even mean? what are the implications of "system instability"?
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No, it's just a warning. You probably aren't going to cause any problems by turning the priority of a process up or down a notch. And there won't be any irreversible damage; anything you do mess up can be fixed by a restart. Just make sure to save your work first before experimenting!
There are a couple of common things that can go wrong (and thus that you should watch out for):
You can crank up the priority of a non-system process so high that it can cause the system to become unstable and non-responsive because the system processes don't get enough time.
You can crank down the priority of a system process so low that it doesn't get enough time, causing your system to become unstable and non-responsive. (Although I think they've fixed some of this more recently by preventing you from altering the priority of certain so-called "critical" system processes.)
In my experience, assuming a decently fast and stable machine, as long as you stay away from the "Real Time" option, you will be fine. Turning non-system process down in priority level is even less likely to cause harm.