Hi Stanley, welcome to Seven Forums.
Give this a go:
When Aero isn't working, right click your desktop and click "Personalise" at the bottom of the window that comes up, you will see an option that says "Troubleshoot Aero and other Transparency errors" (or something like that, can't remember the exact wording, and cant look as my Aero is working fine)
This will run the Aero Troubleshooter. It should at least, give you an idea of what is wrong and it may even fix it for you.
Post back with results.