What to do if the installation of the TAP-Win32 driver fails?
If your TAP-Win32 driver installation fails with a message such as "The system cannot find the file specified", it's possible that your registry is missing the all-important RunOnce? Key. Windows 2000/XP has a bug that it cannot properly detect new hardware by *.inf files, if the "RunOnce" registry key is missing. See this article for more information.
Also, make sure that your system is not configured to silently reject unsigned driver installation.
For more information on managing tap-windows drivers look here.