If I move my hard drive that has windows to a new PC, and boot it from a USB, will I lose my files in that hard drive?
What do you mean by 'boot it from a USB'?
Details, please.
The new pc wont boot from my old hard drive so i have to boot windows (which i obtained from my previous pc) from a USB. So I'm just wondering if I'll lose files on my old hard drive.
Your new system will not boot, as the OS was installed on a computer with completely different hardware.
You need to back up the data stored on the drive (old computer), attach it to the new computer and perform clean OS installation with the OS USB. This would reformat the drive. This is why, you need to back up the data first.
The best means to retrieve data from a hard drive is to clone it to a similar hard drive if you're replacing it. Disk imagers like Macrium Reflect or Drive Cloner Rx can do this.