Variable speed

Hi,

I'm playing a windows video file .wmv on my windows media player 9. It
should last for 764 seconds, but when I record the time it actually uses, it
shows that it uses between 725 and 763 seconds.

The file is stored locally on my harddrive.

Why is this happening? Is there something with the buffer?

My computer is a pentium 2 with 256 MB RAM. Is my PC too slow? If so, why
doesn't the movie use more than 764 seconds instead of less?

.