I just got my first SSD to give more speed to a three year old computer.
I don’t want to put all programs onto the SSD. Can I assign it to A or B, use it for the operating system alone, then leave programs on the current C drive, but boot from the SSD – without updating the registry?
For example, would Excel lose performance from leaving it on the C: drive (which is a 5400 rpm HDD)?
If you are installing your Operating System on the SSD (and it looks like you are) you will need to re-install all applications. This is because almost all applications out there rely on registry information for them to properly function. Running them as you describe would work against you, some would barely work and some may not work at all.
Some examples of applications that do not rely on the registry are STEAM, eclipse, modern Blizzard titles (Warcraft 3 does, but WoW is completely stand-alone, Starcraft 2 is stand-alone).