Getting MSP430 USB Driver Signed By Microsoft
It's not so bad, actually. I've done this a few times at work, writing and signing an .INF/.CAT for a Microchip PIC24FJ64GB002-based USB CDC-ACM-to-I2C interface.


Establish a relationship with a certificate authority. You will need to provide them some basic information which they will validate. If you go for Extended Validation (EV) they will ask for a lot more, but you shouldn't need to go that far. Thawte, Digicert and Comodo are some examples of CAs.
Purchase an Authenticode code-signing certificate from the CA. This is what you need to sign the .CAT file associated with your .INF file. You don't need a kernel-mode certificate for this level of signing.
Install the certificate (goes without saying)
Download the Windows SDK framework and Windows Driver Kit to get the Microsoft tools needed (catalog generation and signing).
Package and sign the drivers:


Use inf2cat to generate the catalog (.CAT) file for the .INF file
Use signtool to sign the catalog file



From this point you could make some sort of driver installer, or use dpinst from the SDK to do basic installation automation.

EDIT: Notes from various comments below:


Creating an .INF file to load USBSER.SYS is not creating a kernel-mode driver - you are using USBSER.SYS (built-in, signed and trusted already), not creating something new - and as such does not require an EV signing certificate and WHQL certification. A basic Authenticode certificate is all that is needed.
Starting with Windows 10, you do not need a signed .INF at all for USBSER.SYS devices, as Windows will pick up USB CDC-ACM devices (Class_02) auto-magically. They have also rewritten the driver from scratch and it works much better now (it even handles unexpected device removals and re-insertions without locking up the virtual COM port, which is a huge plus!). That said, you still can use the signed driver approach if you really want to - the signed driver will still 'work'. There are still lots of Win 8.1/8/7 and older machines out there so having a signed driver is still somewhat important for USB CDC-ACM devices.


Further expansion on your questions:


You do not submit anything for signing to the CA - you purchase the code signing certificate, install it on a machine and do the .INF validation and signing yourself.
You do not need to pay an annual fee per se - however, the certificate you buy will have either a 1-, 2- or 3-year validity period. When the cert expires you will need to buy a new one to be able to keep signing - anything previously signed will remain valid but you lose the ability to sign new stuff.
The .CAT is a hash for the .INF - any changes to the .INF will be detected and the certificate will no longer be valid, which means the .INF would behave like an unsigned one.