Wow spann - your post is as factually inaccurate as my 6 pack abs. The only difference is that when I tell people in a chatroom or forum that I have six pack abs nobody can prove that I dont. Where as.....
A modem of any kind, be it dial up or cable or dsl, simpy converts a signal that your processor cannot understand into a signal that it can. DSL forexample uses carrier frequency's over your existing telephone frequency's. This overlap of different signal occurs at two different speed so if you were to sample the signal at a slow speed you get telephone voice - but if you sample the signal at high speeds and decouple it with the first signal then the remaining signal is your data signal.
By itself a computer cannot seperate the signals appropriately so a cable modem / dsl modem is used to clean off the unwanted voice or tv communication and leave the data part. This in no way what so ever looks at the data being sent, blocks any ports, or viruses.
A Hub on the other hand, (also known as a repeater, or splitter) will take a signal and send that same exact signal to each and every device using the hub. The devices are resonsible to know if the signal sent is for them or someone else and hubs (which only check the input line, and copy it directly to many output lines) also in now way what so ever have a firewall.
A Switch, (or a smart hub) acts like a hub but keeps a table of senders and recievers. When a signal is sent to a switch only the device that the data was meant for gets sent the signal. This reduces the number of data collisions, which is when multiple devices are trying to send but errors occure because only one can send at a time. Hubs have many collisions. Switches have few collisions and allow multiple devices to send at the same time. Switches - although smart hubs - are dumb routers, and also have no built in firewall.
A router, on the other hand, is a smart switch. Not only does it seperate senders and recievers, match them up and allow them to communicate simultaneousely (depending on who the reciever is) but routers also keep track of IP addresses. Switches and hubs dont know anything about IP addresses. A router may act as a DHCP server and allocate IP addresses, or act as a gateway. Routers themselves have an IP and any data that wants to leave the network must first send its data through the router. Most routers have built in firewalls (all home routers do). A router will actually look at the signal being sent, sender and reciever, and it will also perform port forwarding / blocking.
A firewall is a piece of hardware or software which prevents access to a computer on a network on a designated port. One computer, over one internet (also known to the media as a serries of tubes) has a serries of ports for which each program has its own port (in most cases). Right now, your web browser is using port 80 (HTTP) to communicate with the webhost server. A firewall can block some or all ports to a givven computer, and often all internet access in both directions. This however, does not prevent a computer from getting a virus.
The least common way to get a virus as a home user is for a virus to spam itself to other random ip addresses over many ports looking for an open port that it happens to know hwo to exploit in such as way as to download itself without your permission and infect your computer. Most of the time, the things that do this are hackers, and trojans, not viruses.
It is severely more common to get a virus from visiting the wrong website, downloading the wrong file, installing the wrong program, opening the wrong email, or trusting the wrong person. All of these methods of getting a virus will get past a firewall because you as a user give permission for programs to access the internet - such as games, firefox, and your favorite file downloading program. By allowing this you are saying "dear firewall, i want to be able to use the internet otherwise i wouldnt be paying $50 a month for it. Please block every thing that I dont actually use." At this point things which you do use, will have nothing preventing a viruses to enter through their port (unless you have antivirus duh).
While I come to an end, incase you are wondering a worm, virus, and trojan are all different. Trojans dont coppy themselves and traditionally dont harm your system, they just lower your safety settings so that worse things can get in. They are also usually the one who know how to take advantage of loopholes and exploits, using them to upload a virus.
Next of all, your statement of 'avg doesnt slow your computer down' case and point
http://reviews.cnet.com/4520-6600_7-6655968-1.html
You can clearly see that AVG was hands down the AV which affected the computer system the MOST, causing it to be incredibly slower than even Norton (which everyone hates). Myself, I try to be as bypartisan as possible and i make judgments based upon fact and give things equal chance. too bad Norton haters never will
AVG used to be awsome, it was unintrusive, and well equiped to handle threats. But lately it has been more intrusive than any other anti virus out there and according to antivirus comparatives .org, it has lessened in its AV effectiveness.
But hey you are right about one thing which I cannot dispute: AVG does come in a free version. Congradulations.
This is another case of people talkign about things they dont know about. Let me stress that I do not in any way hate or hold any ill will towards people who dont know the answer to things, or dont knwo how things work. But I do dislike it when they go arround pretending they do.
No, offence, but understanding that a firewall is different than antivirus isnt that extremely difficult, but if you want up for round two we could always get talking more about IEEE 802.3 standard. I only bring it up since it is related to this topic.
IEEE 802.3 is also known as ethernet.
EDIT:
Related links:
----------------
http://www.av-comparatives.org/
http://edgegamers.us/e107_plugins/forum/forum_viewtopic.php?138021
http://edgegamers.us/e107_plugins/forum/forum_viewtopic.php?124998.0#post_125604
http://reviews.cnet.com/4520-6600_7-6655968-1.html