Hi RodNorthcott,
CHKDSK cannot make changes to a disk which is in use - which means you will not be able to repair the drive you have booted from, at least not within Windows.
Open Windows Explorer, right-click on the C: drive, click Properties, and use Disk Check from there. This runs CHKDSK under the hood, but gives you the option to schedule it for when you next reboot.
I hope this helps,
Regards,
David W