Sharing Files Results In Access Denied?

I have Xp pro workstation that i set up as a server to share files and folders. I grant specific users both the shared permissions and advanced permissions. they can see the folder and access the folders. but they cannot modify them. It says "access denied". I verified that the permissions were not propogating from the parent folder also. Everything that i see says it should work, but it doesnt. iThe only thing i got to work was giving the user full rights to the above folder..which is for all to share but not to modify.