Where is svn.exe in my machine?

During the installation of TortoiseSVN, check the Command Line Client Tools. This will create the file svn.exe inside the folder C:\Program Files\TortoiseSVN\bin.

The subversion program code is linked into the TortoiseSVN binary. You can install a compatible discrete version if you need to access the repository from the command line.

UPDATE: Recent versions of the TortoiseSVN package can install a discrete svn.exe in addition to the one linked into the GUI binary. It is located in the same bin directory where the main program is installed. In the installer, the command line tools need to be selected for this:

(If you have already installed TortoiseSVN, then rerun the installer and select "Modify")

TortoiseSVN 1.7 has an option for installing the command line tools.

It isn't checked by default, but you can run the installer again and select it. It will also automatically update your PATH environment variable.