The program doesn't start after i plug arduino

I have a small problem sometimes when i plugin the arduino board the program that was on the board does not start and i must plug the board out and in again. Any ideas why this happens ?

Does the power led light? and does the pin 13 led blink when reset?

If the power light does not always come on, I might guess it's a defective power/usb cable

sciguy: Does the power led light? and does the pin 13 led blink when reset?

If the power light does not always come on, I might guess it's a defective power/usb cable

Does the PC 'see' the new USB connection being made when you plug in the board, but it doesn't seem to run the sketch?

Yep is detected if i upload a new sketch it works. What i must say is that when i plug the usb cable sometimes an led on the board flashes when it flashes it works when it doesn't it does not work. What could be the problem ? I read in the general problems that a cause can be if i am sending data over the com port but i am not.Is windows sending some sort of data over the com port ? I use windows 7