If you change 4 major pies of equipment MS consider it to be a new PC
Motherboard, Hard drive, CPU, memory.
HAve you done this?
Just go through the activation procedure by phone
How to activate Windows XP by phone To contact a Microsoft customer service representative to activate Windows by phone, follow these steps:
Click Start, point to All Programs, point to Accessories, point to System Tools, and then click Activate Windows. Or, click the Windows Activation icon in the notification area. Click Yes, I want to telephone a customer service representative to active Windows now. Click Read the Windows Product Activation Privacy Statement, click Back, and then click Next. Follow the steps in the Activate Windows by phone dialog box, and then click Next. Note The number appears now and differs based on the location that you select. When activation is completed and you receive the following message, click OK.
TAlk to the person at the other end thaey are very helpful.