Right click, select Run As Administrator. If your current account isn't an
administrator, you'll get prompted for username and password. If it is, you
won't - just a LUA confirmation that you really want to do this.

If you're in an enviornment where there are system administrators
controlling your workstation, it's possible they've set a policy on your
account that doesn't allow you to elevate. in which case, you won't even see
the Run As Administrator option.