What to do about counterfeit Windows warning

I have a Dell computer that has just started waring me that Windows XP may be counterfeit, the operating system was pre-installed on the PC by Dell. Can I just ignore/disable the warning, or should I contact Dell?

EDIT:
Should I be worried that this may be a virus or malware?

Update:
This PC is 2-5 years old, the OS is original, and no hardware has been changed. The message started to appear within the last week.

I would contact Dell - your Windows key/activation code should be unique.

EDIT: Even though the machine is (probably) out of warranty, they might be able to shed some light on why it's suddenly failed WGA. Microsoft do update WGA periodically and I believe add to the list of "bad" keys/activation codes. It could be that yours is in a range that's been compromised. Dell might be able to provide you with a new one.

Use the WGA Diagnostic Site to troubleshoot this. If that doesn't offer any useful information, post your situation over at the Microsoft Genuine Advantage forum. If they have bugs in WGA, it is good to provide information for Microsoft to fix the problem.