Did you try a discrete LAN card? Even if for only a test.
The "i have to download the definitions and updates manually." is a small clue there may be malware. While I know to uninstall and reinstall apps and antivirus that fail, some won't do that. I can't explain why.
In closing my advice is:
1. Try a discrete LAN card.
2. Uninstall and reinstall that app.
3. Take Grif's advice from http://forums.cnet.com/7726-6035_102-3254948.html
Bob
i have a hp pavilion a6045d pC and the internet connection keeps on disconnecting byitself. i have to remove and insert again the network cable and restart teh PC to get it working. i also have to repeat this a couple of times to try to get it work. i am also unable to update the PC with windows update and antivirus(Microsoft security essentials) i have to download the definitions and updates manually. I need urgent help as some of my software too has stopped working. any ideas? this problem started after i sent it for reformatting at a shop. i also have changed the motherboard and network card to a new one. I needurgent help as i need to fix this by monday! thanks!

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