Please help

When I try to carry out the Scandisc operation I am told that the scan
requires full access and that the scan will be carried out on the next boot
up (this I know is correct). But when during the boot up the PC gets to the
point where it should start the scan I get the blue screen, and I get the
error message "Cannot open the volume for direct access", the boot then
carries on to completion. I don't appear to have any other errors at the
moment except that I cannot enter the start up options at the start of the
boot ie Safe Mode. Perhaps the problems are linked.
Please help.