chkdsk problems

[Hassan]

i set chkdsk to scan at boot for errors, but now
everytime i boot i tries to check and hangs when it is
finished, if i cancel in the first ten seconds it hangs
right there. Every now an then it cannot access the
volume and bypasses chkdsk, as this is the only way i can
access xp. how do i unflag the drive from dirty or
whatever so it doesn't start chkdsk at boot everytime?
Please help i am so annoyed right now i cant reboot.