That's going to be hard to recover from. There are always at least two
folders in the Path variable, but there could be many more, depending on
what's installed on your machine. Only the main two can be inferred easily.

Go to Start > Settings > Control Panel > System > Advanced > Environment
Variables.

In the lower (System variables) area, first look for the entry called WinDir
and make a note of the folder name, which will be either C:\WINDOWS or
C:\WINNT depending on your version of Windows.

If the Path item is there but empty, select it and click the Edit button. If
it isn't there at all, click the New button.

In the little dialog that pops up, make sure the Variable box contains the
word Path. In the Variable Value box, type one of these two lines, using the
one that matches the WinDir value:

  C:\WINDOWS\system32;C:\WINDOWS

  C:\WINNT\system32;C:\WINNT

Click OK buttons until all the dialogs close.

As you use your computer, you may occasionally get an error message
indicating that some program can't be found. That's a sign that you're
missing that program's folder from the Path variable, and you can go back
and add it (separate it from the preceding folder name with a semicolon).