The dirty bit doesn't necessarily mean the disk is bad, it just means the disk is in an inconsistent state such being removed or powered down in properly. Run chkdsk C: /F and reboot and it should be cleared even if it doesn't find any errors.
You can also use a disk editor and locate the dirty bit and just change it to a zero although this is a bit riskier and you can easily make your disk unbootable if you edit the wrong data.