XP Activation code no longer works?

This is probably a silly question but...

Having had problems getting XP Home to install and be stable I think I've succeeded. However, as I messed around with a number of settings I decided to format and reinstall to get a clean start. This seems to be an install too far as the activation code is no longer working. I bought the XP home so I'm the only one with code.

How many times can I "activate" before it no longer works?
Can I do anything about it?
What does it mean if XP isn't activated?

 

http://www.microsoft.com/piracy/basics/activation/

 

Refer to the website the other members give you. I think that you need to report to Microsoft before doing a clean install. Also, never format your hard disk without telling Microsoft if you are about to install an OEM Windows XP. If you are repairing, you do not need to report to Microsoft as activation would usually be successful.

Have a nice day!