"Network not available" 4820TG

I really hope somebody can help.
I got a 4820TG a few weeks ago. Out of the box I was able to connect with fairly little hassle to my 2 other XP machines using either a cross-over ethernet cable or the wireless LAN. (No router - ad hoc I guess?)
However my machine had a faulty pixel so I took it in to see if they would swap it, and left it with Acer while I went overseas.
When I got back they had NOT changed the screen but they HAD swapped hard-drive because they supposedly found bad sectors on the original!?!
That's where the "fun" started because now I CANNOT connect either wired or wireless. I had network techs come out, they fiddled for ages, 1/2 got it working byt not really; and suggested I reformat and reload Win7 on the basis that perhaps the Windows 7 install had been faulty- which I did.
But it still doesn't work. I have downloaded fresh drivers for the LAN and WLAN (found them here: Acer Aspire 4820TG Windows 7 Drivers | Laptop Drivers Download) but that also has not solved the issue.
So I basically have a beautiful 4820TG that can only "communicate" to Internet or to USB devices but not other computers.
I would love to tell them to give me a brand new machine but somehow I don't see it happening - even though THEY SHOULD since it worked fine up until they TOOK IT UPON THEMSELVES to switch the HDD without discussion...
PLEASE HELP!!!?
TIA.