Windows Aero is the default Windows user interface in Vista and 7. Sometimes you'll get that notice if you have some of your Aero settings too high (Or if Windows thinks you have it too high) and will slow down your computer, because you computer has to
"draw" a lot more special effects than the Windows Standard or Windows Classic user interface.
As for running slow, there is a difference between your dial up connection, and your computer's processing power. Your dial up connection connects you to the internet, where the speeds you will notice will be how fast a webpage loads, how fast images load, ect ect. Even though your computer's processing power has a say on how fast those pages can load, your dial up connection will dictate most of it. Things where your dial up connection generally doesn't have a say in something (I.E: Browsing files on your computer) will be much faster than browsing the web, because dial up is very slow, but your computer isn't
that slow.
AS FOR WINDOWS AERO.
Sorry, kinda lost track there for a bit. If you do think that your computer is slow, you can use the Windows Classic or Windows Standard interface design which will free up more resources for your computer to use elsewhere. If you want to change it, go to your task bar and click the Start button, go to Control Panel, click "Appearance and Personalization", then you can probably change some things there, but also click "Personalization". I would imagine you can figure out the rest
. If this isn't right, sorry. I have Windows Vista, and it's slightly different in Windows 7.
