Sinema,
I knew in 2 seconds flat that the site you posted is selling bogus software, because the prices are way too low.
Please do this:
Step 1 is to download and run the utility at this link http://go.microsoft.com/fwlink/?linkid=52012, then click Validation, Copy to Clipboard, then paste the report into a response message in this thread.
Step 2 is to look on the computer to see if you have a Certificate of Authenticty (COA), or look in the materials that you were sent by the site for a COA. If you have one, tell us about the COA. Tell us:
1. What edition of Windows XP is it for, Home, Pro, or Media Center?
2. Does it read "OEM Software" or "OEM Product" in black lettering?
3. Or, does it have the computer manufacturer's name in black lettering?
4. DO NOT post the Product Key.
Not sure what to look for? Click here: http://www.microsoft.com/resources/howtotell/en/coa.mspx