A lot of keyboards are preset to stop most other input once one of the letters or numbers are being pressed. Try setting your horn to the shift key or something and see if it still does that, since you need shift to be able to be pushed at the same time as letters to type normally. Also from my past experience if you use the number-pad keys for steering, acceleration, and braking, it tends to not interfere with what you push on the letters/numbers set of keys. Believe it or not this is an issue that has plagued keyboard drivers for decades, which is how I know about it. A few times in the 90's and 2000's I'd have a controller break on me and have to resort to keyboard driving in the old Papyrus sims, where this would become an issue.... so I learned to use numberpad 4 and 6 for left and right, numberpad enter for accel, numberpad period for brake. But I also know that even today, most keyboards that are not the more expensive type still have this limitation.
It could also be something else, like if you have the "sticky key" features enabled in windows. But try my first idea first.