guess:

there is(are) program running which will change your default setting. Sorry don't have enough time towrite more. use msconfig click startup tab, uncheck any strang process, restart your computer, also you can download AD-AWARE to clean any spyware.
by the way you can post all process you found in msconfig, may someone know how to fix your problem.

have to sleep, it's already mornings.
have nice weekend