Weird title I know but it's rather hard to describe!

Basically I'm running Vista Home Premium as an administrator, and everything was fine until after a few program installs I did (nothing major or anything), the little shutdown menu which used to be:



now only has a limited menu of options with the padlock and the arrow allowing "switch user, logoff & lock". When I try the old Alt + F4 option I get a dialog box "This operation has been cancelled due to restrictions in effect on this computer. Please contact your system administrator."



This, coupled with an inability to run regedit ("Registry editing has been disabled by your administrator") makes me believe I have somehow removed privileges or something. But I'm the sole user and User Accounts tells me I'm an administrator... Anyone got any ideas on how these policies/privileges have been edited and how to get them back to an all powerful restriction-free environment?

edit: Had a look here and found my problem in the tutorial section, and I would attempt the manual way or method 2 but I can't run regedit, and this just emphasis the fact that something is up...

Thanks and I'm glad I found this place, let's hope I work this out.