How can I do to obtain this certificate ?
An example from VerySign http://www.verisign.com/code-signing/content-signing-certificates/microsoft-authenticode/index.html
Will it be ok for the VirtNet source code if I purchage the developer update, since I have no modifications to do ?
Yes, it will be ok.
But if my software runs well and I don’t need neither support nor upgrades, will I still have the right to sell my products using your driver or will I have to renew my subscription ?
No, you have not to renew after one year (and can continue to distribute your software) but you can purchase the renewal anytime when you really need some updates.
Another question, I don’t remember if the driver works with Windows 7 64 bits ; if it’s not the case will I have the new version if it’s released within one year ?
Windows 7 is already supported.