Task Manger Disable Admin

I'm having some problems with my PC, I use Vista home basic by the way. I think its to do with administrative privilages. Basically my account is the Administrator account but some things won't work for me. For example, if i want to change my IE homepage i can't because the field to type in is unclickable. At the bottom of the options page it says "Some settings are managed by your sytem administrator". Another problem is when I try use Task Manager, a message appears saying "Task Manager has been disabled by your administrator". I have run scans for malware and stuff like that but nothing has been identified. I can't remember making changes myself but I do have younger siblings that may have in advertantly.