Could you please provide more detailed information (see my sig) and tell us who your ISP is (or if you’re using BitComet while at university/college/work)?
It is possible that you have a problem with your PC.
I suggest you run a complete hard drive scan (scan disc will do, be sure to do surface scan).
Also scanning for viruses, and spyware is always advised.
Also, be sure you have your client set to do a final hash check after completing. This step is not required, but will find and replace any pieces that are damaged by hard drive failure, or other cause.
After your scans are complete, and all your active torrents are complete, shut down all porgrams and do a full defrag of your hard drive.