Connection problem?

[#1]

When I first boot up my p.c. it will not connect to the internet. When I then open network and sharing centre the diagram shows connection from p.c. to router o.k. but router to internet as no internet access.
I then go to "Troubleshoot Problem" and after the troubleshooter does it's stuff the following message appears "Local Area Connection" doesn't have a valid IP configuration - Fixed.
The connection is then fine until the p.c. is switched off and re-booted.
TG582n router is connected to master socket and directly to Win 7 p.c. via ethernet cable.
If using my laptop wirelessly there is not a problem, only with the directly wired p.c..
All this has only seemed to happen since my connection dropped after about 22 days of stability a 6Mb/s. It then connected a 7Mb/s for a short time and has now re-connected at 6.3Mb/s.
It appears to have "gone to pot" after the increase to 7Mb/s. In 7+ years of using broadband, from the same address, I have never had a 7Mb/s or even 6Mb/s connection even at the change over to ADLS2+ when I thought it lucky to get 5Mb/s! Why couldn't it just stay at the stable 6Mb/s?
My router has been powered continuously, I have done no resets, disconnections, re-connections etc..
Any ideas?

[#2]

Just a thought, but do you have a static IP address set in the PCs network connection properties?

[#3]

I have had a lot of fun with Win 7 recently and this problem.  If you don't choose exactly the right network type when it asks it assumes you are on a public network and locks everything down.
Convincing win 7 to then let you change things can be hell or easy depending on whether there is a gateway address set correctly or not.  If that gateway address is not set exactly right you will have nightmares which kind of does point to why Matt was asking you about a static ip-address.  If it's not static then you are almost certainly not getting DHCP assigned fully and if you run an ipconfig command I would bet there would be a blank gateway field.