Windsor & Eton Central is MUCH closer to the castle, it's on the hill, up most of the way. Just a short walk out of the shopping area (covered, if it's raining etc) from that train station and you should be able to see the castle. If you're already at Paddington, that will be super easy for you. Slough is a very small station, not a big deal to do a change there at all. Not crowded during the times I've been there.
Lots of good little shops, right near that train there's several nice places to eat for good prices as well.
I also took the hop on hop off bus tour too, but I don't know that it was the greatest way to spend time there. I would have rather spent the time in the castle.
The first time I went, I had taken the green coach bus over there, which was a very comfy way to go from Victoria Station to Windsor, it stops at the top of the hill right near the castle gate. So that's another nice option that has no changes, few crowds (usually... unless it's LegoLand time) :)
Have a fun trip!
-Alison