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.