Pushing through the whole install fresh Windows 10 on the new PC and transferring across optical and harddrives and wotnot.
Finding out all the small glitches I hadn't known about - such as having the m2.ssd plugged in automatically kills two of the SATA ports on the motherboard because they use the same channels apparently. Swearing up a storm over the difficulties of transferring things from Windows 7 to Windows 10. Cursing all the annoying automated ****e MS has set up for synchronisation across platforms that I have to spend time turning off, but then having no automated features for restoring windows 7 data to the right spot in Win10 and having to do it all manually. Its just a ridiculous amount of petty, fiddly crap to deal with rather than simple set it up and let it go.
Reinstalling all the odds and ends. I know I'm going to forget something.
I've never had a situation where because of the sheer stress and frustration of dealing with all of the stupid things I've actually regretted upgrading the PC.