It sounds like you have a program with an auto update feature running in the background. It could be some spyware trying to call home but they usually are a little sneaker. When you hit Ctrl-Alt-Delete, see what other programs are running. If you can't tell which one is the problem, end task one at a time until the logon windows stops popping up. That should tell you the program. you will need to see if there is a provision to shut off the auto updater or whatever.

One other item you can check is in Control Panel>internet options>connections. Choose the 'never dial a connection' option, then apply and ok your way out.