Need IT help!

Okay, I'm about to start crying so before I do that, I'll try a last attempt... which is your help guys!
For about a week now, we had a serious problem with one of our computers at home, let me explain:

Setup:
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We have 4 computers: 1 mac, 2 AMD, 1 intel.
Of these computers, to get on the Internet, 3 are wired, 1 is wireless (the intel).
We have a router LinkSys WRT54G v2.0
Of the 4 computers, only 2 are used to play GuildWars and they are the AMD. They have nothing else on them, just Windows and GuildWars!

Problem:
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Nothing in the configuration of the computers changed, nothing in the physical environment either. I did not install any new software. Basically, nothing changed and a problem appeared. The problem happens when I turn on my second AMD (Sempron, ASUS A7N8X-VM/400, 1gig): as soon as I turn the computer on, the Network goes berzerk (lights flashing like crazy) and I lose the DSL line for about 30 seconds. If I wait to get the internet back, I lose it again like 2 minutes later...

Steps:
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To pinpoint the actual problem, I called the ISP tech. support and followed the procedure... Have I installed somehting new? No. Have I moved something? No. Etc. etc.
SOOOOooo... what I did then is this:
- I reformatted the computer
- Tried another router
- Updated router firmware
- Tried another modem
- Updated modem firmware
- Tried another LAN card
- Tried a wireless card
- Moved the computer to another room
- Installed new drivers for everything
- Flashed/Updated CPU BIOS

After all the steps enumerated, I still have the same problem... Now I'm thinking it could be either the motherboard or the processor... but as I'm writing this down, I'm also thinking it could be the powersupply not strong enough (400W for a sempron should be enough no?)

So... erm... any ideas / clues / hints... anyone?

If your network is going haywire and messing around and losing connection. It has NOTHING to do with your Motherboard, RAM, Graphics Card OR Processor. It's something to do with your networking and router. So look around in that.

Wait; infact, is the network going crazy when you turn the PC just on? Or when you load up another Guild Wars?

Definitely a networking problem, I highly doubt it is related to hardware.

Network goes nut when I turn the PC on. When entering guildwars, it simply downloads the 1400+ files at 1k per seconds and after 30 secs, I lose internet again. Whats on you mind guys? Any settings in the router I should change?

What happens to the network if that "problem machine" is the only one powered on and connected? Does the network still go haywire?

Okay, here's an update:
- Changed the cables, didn't work
- Also changed ports on the router, didn't work
- Plugged the computer straight to the modem, the DSL still goes nuts when turning on the PC
- Called ISP they say the tests they ran on the line didn't show up anything unusual

Still screwed

Okay, final update: I finally found whats causing all the mess... I've been unmounting the PC piece by piece to find out that there was only the mobo and the processor in the case. I was still losing the DSL so I took everything out of the box thinking it could be a shortcut... nope... so I removed the last peice: the CPU. Now guess what... the mobo can start, stay on, resetted and I don't lose the DSL line. Tried that 20 times (counted) and it nothing failed!

Is this crazy enough for you or what? I thought it was impossible! So I opened my other PC, switched the CPU and turned on the PC. Result: lost DSL line!

Now how on hell a CPU can interfere with network or DSL, I have no clue. But now that I finally found what the problem is, I can finally go to sleep... welp almost: how long is an AMD Sempron warranty? Heh.

Later folks!