Clearly something wrong with the motherboard.
I'd try to replace the 4 sticks of 1GB by 2 sticks of 2 GB.
Here's the configuration of the computer.
Acer Aspire M5100
Motherboard: F690GVM (that's what Open Hardware Monitor shows)
CPU: AMD Athlon 64 X2 Dual Core 6000+
RAM: 4 sticks of 1GB DDR2 NANYA NT1GT64U8HBOBY-25D (looks like these are replaced as there's HP stickers on all of them, besides, I can read 3GB of RAM on the box)
HDD: 500GB SATA II Seagate Barracuda ST3500830AS
Video: Onboard ATI Radeon X1200
Windoes 7 (all updates installed)
Computer fully scanned with Security Essentials and Malware Bytes
With 4 sticks of RAM installed, LAN won't work. With automatic IP, it shows the received IP as 169... , although my network gateway is 192.168.1.1. When I set IP manually (something like 192.168.1.150), it still doesn't work. No ping, browsers or anything.
Also instead of any sound you'd get a static noise. By the way, all of these is not mother board position dependent yet.
It all goes away when I remove the RAM stick in the 4th socket. A particular stick does not matter, It just has to be 3GB to get working. I have shuffled them. Moreover, the issue persists when I installed another 2 sticks of 2GB (total 4GB) from a different computer.
Now I'm telling you about the issue with 3 sticks (3GB in total) of RAM installed. OK, now check this out. Whenever I lay the box on it's side (horizontal position) everything is cool. When I put it vertical, it's normal position, I start getting a lot of ping time outs, widows update ends up throwing error 80072EE2, Skype, Team Viewer are offline, although the network sign in notification are show no problems. By the way, I'm sure it's MoBo since I have removed every single component, including MoBO, from the box and have twisted all possible ways. Only the motherboard twist from horizontal to vertical affects it.