Thanks, I'm primarly trying see the harmonics on a AC line. Then I want to see if that passes on to a computer power supply, and ultimately to the USB ouput.
I don't probe mains... Even if it is full of trash, there's probably nothing you can do about it.
You can safely look at the power supply output if it is a normal PC because the low voltages are referenced to the chassis ground which is earth ground which matches the earth ground on the scope. Same with the USB power, one side will be earth ground. There is an exception if the PC only has a 2 conductor power cord or it uses a power brick with a 2 conductor plug.
You need to think very carefully about how you probe stuff. If is often better to check things with a DMM. For example, check the voltage between what you think is ground on the PC and an actual earth ground like the BNC connector on your grounded scope. It should be 0V or very very close. If it isn't, you can't hook the probe ground lead to that point in the circuit.
Dave did a video on "How Not To Blow Up Your Oscilloscope". Watch it a few times:
https://youtu.be/xaELqAo4kkQBefore you start probing power systems, you need to ask yourself "why". What do you hope to learn? What can you do if the display has trash? Is making this measurement worth a) my life (in the case of mains) b) my scope and c) my power source. If you mess up on the USB connector, you can take out the entire motherboard (in the worst case).
Why do you think the USB power is noisy? It probably isn't, you know.
I look at signals on PCBs and breadboards and they are usually powered by a wall wart or floating power supply. The voltages are low and I like it that way. My idea of high voltage these days is 12V. No more of the high voltage, high current, industrial systems. I'm retired!