Start>Settings>Control Panel>Users and Passwords>
Double click to see a list of defined users and their permissions. The logon you are using should be listed as being in the Administrators group. Is the logon you're using the only one on the machine? If you need to add a user or change the permission, go to the Advanced tab and click on the Advanced button. This will take you to the screen where you can add users or change their permissions. To add a user, use the dropdown under Action. To change permission on an existing user, right click on the user and look at properties. Go to Member Of, and make sure the group is Administrators. Use the Add button at the bottom of that page if you need to add the Administrators group.
Let me know how it goes.