I have Windows 7 on both my desktop and laptop. Only on my desktop does this happen: After connecting to the internet several times (the number and time intervals vary randomly) I lose the ability to connect — either via Firefox or IE. I restart my computer and I can connect. This does not affect email or other connections to my isp. Since this never happens on my laptop (using the same router), I have ruled out an isp or router problem.
Herb Reed