No. Some pcs use a usb connection, like laptops. Others use wireless.
To check for a network (Ethernet) card, for me it's Start->Settings->(Ctrl. Panel)Network Connections and under the heading "LAN or High-Speed Internet" should be a title eg. (Local Area Netwk.)
Then look to the right under the bar that reads "device name" that's your card. (mine reads: Intel(R) PRO/100 VE Network Connection)
Bear in mind, finding the Network Connections Window may differ because i'm not using WinXP.
So a network card is required only to connect to the internet using the ethernet connection; and provided the pc has an ethernet socket then it probably has a network card?