Unable to defrag and scan disk problems

I wrote in earlier but have come across soemnew things. I am running win 98se. I am unable to defrag. It says I have errors on my drive. I try to scan disk and it says I need a thorough scan disk. I do that and it finds no errors. Anytime I start orreboot my comp. it tells me windows shutdown inproperly and does the scan disk and then proceeds to a thorough scan disk and finds no problems. I have a western digital hard drive and have ran the diagnostic disk on it an it says the disk is fine. Also I am getting alot of illegal operations and the comp. freezing on me.
David