A 500 GB USB portable disk drive (at $50 or less for 500 GB) probably costs less per GB than the biggest USB flash drives (64-128 GB). The Windows file transfer program that comes with the new PC can usually package everything you had on the old one, then unbundle it to the new one. You can transfer with a USB transfer cable, or put it all on a drive big enough to handle it.
But what I've done on my most recent transfers has been to set up an intranet so that both PCs share their files and just copy across the network. Even the restricted "home" versions of Windows permit file sharing, if what you want to do is move or copy specific folders PC to PC. I've made this work for PC to Mac, which have quite different file systems, networking protocols handle the differences.
Network mechanisms? Hardwired ethernet is fastest, especially if your router and both computers handle 100 gb/sec or even 10 gb/sec rates but even WiFi (at 1 gb/sec) will be faster than writing and then reading from USB drives or memory sticks.
If you don't have a router, Windows, in versions since XP at least, will let you set up peer-to-peer via WiFi between any two computers.
