i've just bought two brand new laptops (one fujitsu HDD sata2 WD 320Gb 4200 rpm and one DELL HDD sata2 seagate 320gb 7200 rpm); when i scan the hdd's with defraggler the program shows bad clusters (see attachment) but manufacturer tools (seagate low level DOS and WD) to find and repair errors/bad clusters, find no errors.
Get a tool that can read your SMART information, like Speccy, and check what your Re-allocated sector count is. This will show you a figure that relates to the amount of bad / flagged sectors that your hard drive had to evacuate whilst transferring / writing data.
If you're really paranoid, you can try running a full diagnostic with the Western Digital Data Lifeguard Diagnostic tool, or you can run chkdsk on the volume with:
chkdsk letter: /r
example:
chkdsk c: /r
You should get a prompt asking you if you wish to lock handles, if you answer no, you will be prompted with another question asking if you wish to schedule a chkdsk during the next boot. Press Y and restart your computer. Observe the results routinely to see if you have any bad sectors / indexes / clusters being repaired.