The kernel is the most basic instruction set in the computer. In a nutshell, it handles all of the communication between all of the software and hardware in your system. It will always be running and will be higher when background processes are running (indexing, defrags, virus scans etc), but if it is always running that high, then it could be anything from a bad driver to a bad program. Your best bet is to think of when the slowdown began and think of anything you changed in your system (driver update, new software, new games, trial programs etc). Keep an eye on what other process are running.