Cannot add a computer to a workgroup

I have a Dell Prcision 340 workstation. I recently re-installed Windows XP Pro because it was performing slowly and it had been a few years since the OS had been installed. It was shipped to me with Windows 2000. I had to remove the modem because of a yellow question mark in the device manage on “PCI Communications Controller”. When I removed it the Question mark went away. Now I can’t add this computer to the workgroup on my home network and Outlook won’t let me read or delete messages with a header on them. Can anyone help?