De-frag problem

I am running my pc using Windows XP - However just lately I have been unable to utilise the defrag option.

When I try I get the following message :
Initially it says that Disk degrafmenter has detected that Chkdsk / f has been scheduled to run...please run


Then when I try to run this program it says ....
Type of file is NTFS cannot lock current drive

CHKDSK cannot run becuase the volume is in use by another process. Would you like to schedule the volume to be checked the next time the system is restarted

However when I re-boot the system starts up normally without any chkdsk.

As I download quite a lot of music & stuff and delete I am sure it is only a matter of time before the PC starts to slow up.

Nothing is scheduled to run except my symantec anti-virus prog...so not sure why it thinks it is scheduled.
I have tried closing down anti-virus / zone alarm / windows defender etc...but to no avail.

ANy help is much appreciated lads