DOS had a path environmental variable, in a file named "config.sys". Windows has something similar, sometimes it uses config.sys, and sometimes it uses another file, depending on the version. The registry also has data on this.
Best answer is to look up PATH in your Windows help, which will have the right info for your version of Windows. In some versions, you can change your PATH (careful now!), right in Control Panel, look for the System >> environmental variables tab, iirc. In Windows 7 it's:
Control Panel >> System >> Advanced System Settings >> Advanced Tab >> Environmental Variables button.
Thanx Adak. That helped. What really are these Environmental Variables? and purpose do these paths serve>
Environmental variables tell the OS where your OS is found, and other system info, that it needs, while it's booting up (and uses from then on). The PATH for instance, tells the part of the OS that is booting up, (which it finds from the master boot record), where to find the rest of the files - programs for the dll's to run, and all the so-called "external" commands of Windows. (External commands are commands that either use a lot of memory or are used less frequently. The OS doesn't keep these in memory, but loads them only when you need them.)
If the Turbo C directory you're running from is not in the path, any system() lines of code will crash the program it's in, or be ignored. All depending on the phase of the moon, and the lottery numbers that week. (in other words, it's undefined).
In WindowsXP, (and probably other versions as well), it's a bit confusing because it has two command shells: command and cmd. Command is the normal Windows command shell, while cmd is the console command shell, which can work with DOS sized programs of 16 bit. Each one of these has it's own separate configuration file ("config.sys" for cmd), and autoexec.bat (for cmd), file. In addition, the registry has PATH and other configuration info, stored in it. Windows 7 will not run the 16 bit DOS type programs, however.
Be very careful if you edit any of these settings. There are restrictions on these variables, and your system may not even boot up right, if you have the wrong settings in place.