Before getting into why you shouldn't upgrade to Windows XP, let me start by mentioning those who should do so. Anyone still using Windows 95 or 98 should upgrade. Anyone using an older, 16-bit Microsoft Corp. operating system (DOS or Windows 3.X) should buy new computers and upgrade. And anyone masochistic enough to be attempting to use Windows Millennium Edition on a business desktop should immediately upgrade.
XP is Microsoft's attempt to build a consumer operating system (the Windows 9X and Millennium Edition line) onto the Windows NT/2000 core - and they've succeeded. But if your users are already running NT Workstation or Win 2000 Professional, they already have that core and XP doesn't add any "must have" features. In fact, it gives you a few you might put in the "must not have" category.
According to Microsoft, "Windows XP Professional meets Windows 2000 performance" - it isn't any better; it's just as good. So if you have Win 2000 already, there's no improvement.
Maybe you want new features, such as better support for MP3 audio, easier home video creation, better support of home networking or built-in support for high-speed Internet access (guess your Gigabit Ethernet to the desktop isn't sufficient). See the trend here? This is an operating system designed and built for the home user - not the business user.
Other features being touted for XP include better support for transferring files via a null modem cable; a digital photo editor and organizer; and better support of separate desktops for each family member. And then there's the hotly debated Windows Product Activation (WPA) and the dangerous Remote Assistance feature.
WPA isn't really something with which to concern yourself; it's mostly for the home market. Businesses under volume licensing agreements don't have to go through authorizing every PC and then reauthorizing when hardware changes are made.
Remote Assistance, though, really scares me. I'm well aware that NetMeeting on NT 4 and Win 2000 allows you to let another user remotely control your PC, but only with your participation while you are watching. Remote Assistance does not require you to be present when someone is accessing your computer or to run a specific application in order for the remote computer to attach to yours. Remote Assistance permission can be granted via e-mail - and we all know how secure Microsoft's e-mail products are. It requires Microsoft Instant Messaging, so I urge anyone who is running XP to remove this dangerous (and time-wasting) item.
By the way, Microsoft's license for XP lets you purchase the new operating system, but install Win 2000. That may be the best way to go.