At least one person found that having a specific program running in the background (used to allow control of multiple networked computers with the same keyboard and mouse) caused the mouse to be extremely sensitive controlling the camera, etc.

Hit the corresponding number on the keyboard to use a one of the hotkeyed circles. I don't have the PC version, so assume you have to pause the game first, or open the inventory / skill window or something to configure them.

As described in the manual, on the PC the default key to interact with a highlighted object (open a door / chest, pick up a key, etc) is 'E' or right click (point the centre screen crosshairs at something to highlight it, if you are close enough). Left click to attack, and use the F1-F3 keys to switch between different weapon sets (a bow is quite handy at the start of the game even for a predominantly melee fighter, to draw opponents to you and get a bit of damage in first).

You can select a voice for your character to make comments (during combat or finding good/crappy loot, etc), but if all of the character's speech options were voiced that would greatly increase the amount of voiced dialog, which would greatly increase the costs of localization, etc. Considering you have to read the responses first anyway in order to select an appropriate one, many people would probably skip through a voiced repeat anyway.

It is by design that there is no mouse cursor when moving around or during combat, just in the inventory, etc, where it is needed. The controls had to to work similarly for ground and air based movement and combat, and after trying various schemes, Larian decided that using the keyboard for movement and mouse for camera control worked best.
Hit F to pause the game if you want to check or adjust stats, skills or inventory during combat. At least with the Xbox version you could bring up the main menu and go into whatever you wished, but without pausing first that would give opponents an extra second or so to attack, as you are distracted entering and exiting the menu (which may or may not be a problem).