Dear Juan Carlos, I had the same problem, so I went to Start, Control Panel, User Accounts, Change User Account Control Settings and set it to the lowest (never notify), and my problems went away. I hope this relates to the problems you're seeing.
UAC is a pain in the neck, and seems to exist to allow MS to deflect criticism of the virus vulnerability of Windows by blaming the victim (us). There's a page on MSDN about UAC(http://blogs.msdn.com/e7/archive/2008/10/08/user-account-control.aspx), and their response to HUGE criticism has been to say (1) There's nothing wrong with UAC, and (2) They're going to fix it. Typical.
Les