(computer talk and stuff. might be annoying and terminology-ridden.)
EDIT:
Damn you, awesome user. Why must you finish researching the solution yourself just after I finish typing my tl;dr post?
This is an interesting issue, because this will pretty much mess up the operation of every .bat file I've made so far (and probably some others'). I'm pretty wary of changing the standard execution path to c:\windows\etctoolongtotype though, since the user's OS might not be installed there. (if you actually have your operating system install / windows folder somewhere other than the C: drive, that's a slightly different barrel of fish too).
Path is a system variable which lets cmd.exe know what directories to look in when you type in commands; this is useful because you don't have to type the full filepath to the program you're running (as kendjin's file does). In this case, it's not including C:\Windows\System32, so it doesn't find the program reg.
While you could of course just continue to search and replace "reg" with its full path for each batch file you run, it's pretty inconvenient. If you feel up to it, try changing the environment variable PATH (capitalization varies). Doing this with the windows GUI is probably best: go to Control Panel>System>In the left sidebar, "Advanced system settings">Advanced tab>At the bottom there is a button called "Environment Variables", click>find Path in system variables>double click it>make very sure not to delete this line, cancel out if that happens. Hell, you can back it up in some text file too. > At the very end of the line, add "C:\Windows\System32\;" without quotes, with semicolon.