Vista - UAC Problem or error, please help !!

Hello,

i have a new acer laptop with vista preinstalled. I have defined myself as
administrator and with my user-id i was able to define other users and to
install programs.

However since yesterday i can not do those admin functions anymore. In
control panel i still see that vista sees me as administrator but if will
click on "Add or remove user accounts" nothing happens, for some seconds i
saw a circle is turning meaning vista is busy, then the circle stops turning
but i remain in the same screen. The same will happen if will click on "Set
up parental controls for any user".

Also, i can not install new programs anymore. If i will insert a cd with
software and click on setup.exe then an error window will be displayed with
the message: Windows can not find E:Setup.exe. Make sure you typed the name
correctly. Actually the file is there because i just click on the explorer
menu.

Also, on control panel, if i click for example on "Edit the system
environment variables" then i will get an error window with the error
message: Windows can not find
c:\Windows\system32\SystemPropertiesAdvanced.exe. Again this file is present
in c:\windows\system32 so there is another error but what ?

Any feedback will be appreciated,

best regards
irw777