Please answer most of what you can here:
What version of windows?
Do you get any error messages from AIM or any other programs?
Do you connect to the internet directly or through another PC? For example, your network cable is connected to the router directly and then to the cable modem? Have you had any special rules setup on your router before? Can you try to connect directly to the modem and see if it works then? What is your ISP?
When going to the command prompt (start > run> type in cmd) can you ping google.com and other sites? (ping website.com)
Go to your network connections.. (in the control panel, if you use classic view) can you tell us what you see? Your LAN connections specifically? Perhaps you can toss in a screen shot (make sure you expand the columns so we can see, and please use "details" for the view - menu VIEW > Details).
Finally, did you install all your drivers ok? Right click on my comptuer > properties. Click the Hardware tab, then Device manager. Any red or yellow icons? Screen shot for us?