avi files not playing smoothly
 

I have a large number of avi files all created from various digital cameras over the last few years. They're all stored on my c: drive and have played perfectly up till about 2 weeks ago. Now whenever I play them, the video slows down, then jumps forward to catch up with itself. The audio plays perfectly. If I copy the avi file to my d: drive and play it, it plays ok. This has lead me to think perhaps my c: drive is on it's way out, but everything else doesn't show any hint of a problem. mpg files play ok, the disk is 110gb in size of which 70gb is free. It's been de-fragged. I've even ran a Western Digital utility to check the disk and it found nothing wrong. All codecs must be ok as the avi files run ok from the d: drive, and the same with memory.
Any ideas?

Thanks in advance.
Dave