Vista - No more administrative powers

Hello. I've had my laptop for a little over a year now, at for the first
couple months, everything worked fine..But now it seems I have no more
administrative powers. I'm the only account on the laptop however, and I've
never made any other accounts. Whenever I try to start a new program or do
anything with the administrator symbol on it, a message with the dreaded red
X pops up saying "The requested resource is in use." This is preventing me
from setting up newly downloaded programs, or doing things such as messing
with user accounts on control panel, because they all apparently need
administrative access. Does anybody know how I can fix this?