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I am trying to access the internet from my leopard macbook using internet connection sharing on a vista PC. I can't browes the internet. What am i missing?

I have been using the internet at home on wireless with no problem until about 8 weeks ago when the messages ...

"Do you want the application "configd" to accept incoming network connections"
and

" Do you want the application "mdnsresponder" to accept incoming network connections"

...started to pop up. At some point i may have clicked no to this but for a while it was just an annoying procedure which meant network diagnostics and entering my password every time i opened Safari.
However, the situation now is that i have airport signal at full strength, my home internet name comes up on the list of available networks, but when i open safari and type in a web address it says

"You are not connected to the internet"

When I perform Net Diagnostics is says that

"Your network configuration has changed"
(i am administrator and have not changed it and everybody else in my house, macs and pcs, don't have this problem)
I type in the usual WEP password but it is not recognized - still no internet.

I've tried turning router off then on during net diagnosis - nothing. I called provider, they talked me through disabling airport and using ethernet which worked that day but not now after it's been off. also resetting Safari and clearing cache (some have not gone)

Have also found my Date & Time settings completely changed on a couple of occasions - changing them did not fix problem.

This may have all coincided with me synchronizing my new iPhone to ibook, the installation of a new mac desktop to the network or installing Microsoft Office for Mac- it all happened around the same time.

Anything spring to mind???

Thanks in advance.
Loz