If you decide you want to get a router then you have two options, with wireless or without.
The cheap one Cammy showed you is without, for a wireless one you will need to be around the £60 or more mark.
If you can easily get a wire to both the pcs that you wish to network and you dont envisage having a laptop connected by wifi at a later date, then you may as well get a non wireless one.
Whichever you decide to go for you need an ADSL modem/ 4 port router, with or without wireless. This will allow you to get rid of your present USB modem.
There are some routers that will work with a USB modem but they are more expensive than getting a modem/router combined and you are better off avoiding USB wherever possible.
You are still better off with an ethernet pci card in your ME pc, rather than the additional expense and complications of using an ethernet to usb adaptor.
All you need to do to add a PCI card is usually to remove two screws from the back of the case on the right hand sode, looking at it from the rear. Slide the side cover off. Look inside and see if you have a spare pci slot, theses are white plastic bits about 3 inches long at the backof the motherboard. They just push into place, (sometimes you have to apply a bit of pressure as they can be a tight fit) and are held in usually by a single screw. It may or may not need drivers installing from the cd or floppy that comes with the card, XP usually installs them without any extra drivers but ME may need a driver
The numbers that you will often see are 100/10 mbs. This means that the router/card will auto detect the speed of the network which is 100mb per second on newer ones and only 10 mb per second on older ones. I have never seen a network using only 10mb, although someone did offer me a 16 port 10mb hub recently :roll:
The other numbers will be the ones referring to the wi fi part. These are usually 802b, 802g etc and are the speed of the wifi, b is 11mbps and g is 54mbs. There is another one out now that does 108 mbs but I cant think what the number for that is