Some great features that you can do in Windows XP revolve around network connections. In Windows 2000/2000 Professional to see your IP address, Gateway, DNS, etc you needed to go to the command prompt and manually type in the "ipconfig" command, but in XP there is a great feature if you double click on your network connection while you are connected and there is a new tab labeled "details", it provides you with the same info with simply a double click instead of having to run the command prompt.
In addition, there is another command that you used to have to run in the command prompt; "ipconfig /release" and "ipconfig /renew". These two separate, but similar, commands have been combined into one and have been placed in the details tab of your network connections. The way to utilize this shortcut would be to press the "repair" button. Pressing the button will release your IP and ask the DHCP server for a new address and lease. If no DHCP server can be found Windows XP automatically configures your IP address using "automatic private IP addressing" (APIPA) which gives you an IP address within the 169.254.X.X range.
One more great networking improvement is the "Network Bridging" ability. With this tool you can take multiple installed NICs and essentially assign them all the same IP address. This really comes in handy with "virtual" NICs created by programs such as VMWare.
Some features which are unique to Windows XP can only be found in "Windows XP Professional". These features include Simple Network Management Protocol (SMTP), client services for Net Ware, simple TCP/IP services, and the multiple roaming profile(s).
One new feature that I think is very useful is the built-in software firewall and security it provides (ICF). It prevents people from accessing your computer with out permission. It also prevents common hacking attempts such as port scanning and certain ports not in use from being accessed by a malicious user.