The 50% cpu usage means that there is a program running in the background that is using 50% of your cpu power. It should alway's read 0% in your task manager.
Go back into task manager>crlt>alt>del, and you should see what program is using up your cpu power and memory. In the lower left it should say 0%. If it say's 50%, turn off the resource hog. It should always say system Idle process (under cpu) 99%. This is normal, but look at what other program is running under cpu!!