What I have done on a few occasions is to un-assemble the pc, put the case with only PSU left inside, in my suitcase and then fill/cover it with clothes.
CPU, Mobo, Ram, Optical and GFX all nicely wrapped in bubble wrap in a backpack/cabin case.
Worked fine a year back but perhaps with the new regulations they won't accept you bringing on all this hardware in the cabin?
I've also seen on several occasions how people are bringing stuff which the airlines have not prohibited in the cabin, yet the staff in security are having a bad day and are making life hell for innocent people.
My last incident was a week ago at LHR going to CPH (EU flight) where I had nicely packed my stuff in cabin case yet was pulled aside as I had perfum in my hand luggage. The regulation clearly states bottles under 100ml are allowed.
I kindly informed her about this but she warned me I could be grounded...LOL.
Bag sent to check-in and I rushed to the plane. This day it happened to be nearly 4 hours late...do you think my bag made it to CPH? no
Considering this perhaps the best option is a to get a quality flight case and courier the thing. Although a PC in a flight case will weigh plenty thus costs will soar.
Perhaps Ill just get a PSP?