Just in case, right-click 'My Computer" go to "Manage" -> "Local users and groups" and into "Users" Now, right-click the account(s) that lack permissions go to Properties, and in the "Member of" tab make sure they're members of the Administrators group.
What is it exactly you're sharing on your LAN/WLAN? Is it a printer, file, or other resource? The problem could lie in NTFS permissions of the resource itself. Go to Windows Explorer, right-click whatever it is you want to share go to Properties->Security, and make sure the group your account belongs to is listed there. If not, click the 'Add' button and type the name of your account's group. You may need BOTH your account name and it's group, BTW.