Hard Disk Overload And System Hanging

I bought an acer 4830TG in august 2011. Here's the description: AS4830TG-2434G75MN | Product Model..Everything worked fine with it. It was a very good machine... So when it was stolen from me I bought the exact same model and things went sour:I burned the recovery discs and subsequently installed a fresh Windows 7 Ultimate x64.Then I carefully installed all the necessary drivers etc... Exactly as i did with the first one.The problem is that every time i start an application that read/writes to the hard drives intensively, everything slows down, hangs or crash.When making a copy from my desktop computer over the network (directly through a crossed cat5e cable), the throughput initially reaches 50-70 Mb/s and rapidly comes down to 10-15 Mb/s. At this point, I can't to do anything with my laptop while copying in background... My browser is unresponsive for several seconds at a time, then is responsive again, then is unresponsive etc... If I try to play a video with VLC it seems it can't buffer more than 10 seconds of video before buffering again for several seconds (I solved that particular issue by moving the video file to a ramdisk prior to playback)... Simply opening My documents or the control panel takes more than a minute.

On rare occasions, the system crash and I get a BSOD. Note however that if I let the copy finish without doing anything else, everything works fine except for the low throughput. I know I can expect more than 15 Mb/s because with the first 4830TG it was generally over 50Mb/s.This kind of behavior also occurs when I'm burning a disc, when �torrent is checking a file, when I'm copying files from one partition to another etc.I can't figure out where lies the problem:

- all drivers are listed as "up to date" by windows update and there is no unknown device listed.
- RAM usage appears normal, the system is not overloaded with unnecessary applications. (At boot it uses about 1.2Go with AVG 2012 free, daemon tools, Malwarebyte's and dropbox)..