my computer is running 3 times slower than normal

hey folks im not to bright when it comes to computer, so i need some advice. my computer is running at least 3 times slower than normal. i have defraged. checked for errors. checked for spy ware, and did a virus scan...
how can i find out how fast the hard drive is suppose to be writing and how fast it is writing.
where i notice the problem the most is when i burn movies. it did take like 20 mins now it takes like 2 hours.... any ideas......