So there is only one user account on the computer? If so, it is an
administrative account, and you are logging in as an administrator. However,
Vista's ordinary admin accounts normally operate as regular users, and then
something called User Account Control is invoked to elevate privileges to
full admin status as required.
Maybe your problem is not the account with which you are logging on. If you
post precise examples (including the steps you take) of what you are unable
to do someone may be able to help you.
You should understand that some software is not compatible with Vista. If
you are trying to use software which attempts to make changes to areas of the
system that are protected, and if that software isn't properly asking Vista
for permission to make these changes, then that could be the source of your
trouble.
There are people who turn User Account Control off because of this, but I
think that's a very ill-advised action to take. The operating system was
designed to work with UAC, and it should remain intact. (my opinion)