My first thought would be that there is either something on your profile, or the security settings for you user. Is you User in the administrator group? Right click on "My Computer" and choose Manage. Expand Users and Groups and then choose Users. Double click on your user and choose "Member of" and see if "Administrators" is in that list. If it isn't, Sign in as administrator and add it.

If you are already an administrator, try creating a new user with admin, sign into that and see if you can get on.

Later