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This problem is a butt scratcher.
What happens is the Ethernet port has some type of instant malfunction and kills my DSL modem, causing the modem to reboot. Siemens SE567 combo Modem/Router.
I was running with a Dlink Ethernet card in a slot, so I pulled it out and went back to the Ethernet port on the Mother Board. Problem went away for a week or two then reappeared. About twice a day this is happening(not every day)...
 Note That I have put in new hard drives and reinstalled the operating system of XP SP3 a few weeks before this second round started. Problem was happening about Oct 1 2008. And then disappeared after about a week. So now it's Late December and it has come back... Compaq Trashario. 7 years old.....  And sometimes this week coming back from standby, I have to do a 'repair connection'.. Otherwise no problems ever with Internet speeds or browsing.
My two other systems don't cause this to happen(Vista and Ubuntu Linux).It's the XP box to blame.
No errors in Zonealrm or Avast.. And bug free for ever... Other than the 3 crapwares that came pre-installed on the system..

Hope it's not a Mother Board problem....... Theres no guarrantee that this system will reboot with a new Mother Board, since it's a OEM version of XP..


error logs.............

First happens..
Source: Tcpip
Event ID 4202
System detected that network adapter\device\Tcpip was disconnected from the network. And adapters network configuration was released. If it was not disconnected, it may of malfunctioned..

Then second...
Source: Browser.
Event ID 8033
The Browser has forced an election on the network\device\net Bt.Tcpip. Because a master browser was stopped.

Then third..
Source Tcpip
ID: 4201
Network adapter connected.

 What exactly are you doing when you lose connection ?

 Is there a specific reason for the static IP ?

So I has figured it out. 

Seems that in this Siemens SE567(DSL modem/router with wireless), there in the wireless settings. If I leave on "enable auto scan on power on" , that is causing the crashing(A default setting). A quick test of it was to click "auto scan"(new channel), and it crashed within a few seconds to reboot.
Seems to be posted as a firmware flaw, but only affects a certain number of these units.
Something so simple, yet had this unit for 16 months and no major problems other than these two time frames.
So last night when I was surfing/streaming a radio station, it was rebooting roughly about an hour or so apart and the log this morning showed that it had rebooted about an hour and a half before I woke up the XP machine.  And so far 5 1/2 hours and no reboots. If it happens again, then new unit will need to be ordered.

But figuring it out, sure beats playing with tech support. 

No. Still stock firmware.
 I've only changed the stock passwords and the usual other security settings. Dropped the range when I got it, so it only stays within the house. Haven't dug too deep to see why the 'Auto-Scan' crashes the thing. Or why auto-scan has not caused it to act up all this time before.