This can be a complicated issue. Let's address the 3 most common causes.
To install the virtual drivers, the installer needs to have access to the System32 folder. Some antivirus packages don't allow programs to access that folder. This can be even nastier when the user has disabled any alerts from the AV software. It's a good idea when trying to install DaRouter to turn off any AV software before you try to install.
Another common cause can be not having administrator rights. If you're trying to install DaRouter on a computer that you are not the admin of (like a work computer), OR if you aren't running as an admin on your own computer, then you need to run the installer as admin. You can do this by right clicking on the installation file and selecting "Run as..." and selecting an account with admin rights.
Finally, sometimes the settings for your Driver Signing can get changed. This can cause Windows to not install any unsigned drivers. If the first 2 issues didn't fix your problem, go to System Properties > Driver Signing and check to see if it's set to BLOCK. If so, please change it to IGNORE.
(the above example is in Windows XP)
Once it's set to IGNORE, please try the installation again.
In Vista, it's not as easy. There are 2 main ways to get unsigned drivers on your system. If you press F8 during system start up, you'll be taken to a boot menu. You can select the option to turn off driver signature enforcement here and continue booting, but you'll have to repeat this on every boot when you want to use the driver.
The second method involves using a program like VistaBootPro or ReadyDriver Plus 1.1 to alter the boot sequence and enable this option automatically on every boot.
If you've tried the above and are still having problems, please contact Stanton support.