when i click start and click on the turn off computer button the computer freezes when i bring up task manager and move the task manager around it erases that desktop making it white. but after a few minutes it loads the shutdown menu and i can shut my computer down

Whilst you're looking at the task manager when it is in that state, look at the "processes" tab and see what service is using the most cpu - that's why it's hanging..

Also, when in Task Manager:
If you have a load of processes running and you can't easily see, click the Mem Usage title to sort it ascendingly or descendingly.