Okay, my problem was solved. It was something I never expected, but whoever installed the extra 2 GB ram module did it incorrectly. I did memtest, and went for lunch, but when I came back, I saw my computer sticking a BSOD right in front of my face.
That's when I called a neighbour of mine over. He took apart the computer, and I was shocked to find that the 512 mb memory modules were in different coloured slots (one was in a blue slot, the other in a white). So he rearranged the ram, and surprise, surprise, my computer was working again. As for my mobile bradband and sound issues, i fixed them as well. I think it was a compatibility issue, so i fresh-installed Windows 7 and, voila! There's sound, and my broadband is working perfectly fine. There are no hdd errors, so I am happy again. (The miserable handiwork of that technician which installed the ram
Thanks to everyone for replying to my issue, and I hope that i will talk to many of you again in another forum!