Install Windows Phone Developer Tools on Windows XP

• Download Windows Phone Developer Tools RTW
• Open "Command Prompt" and type the following command to extract vm_web.exe file:
  

  vm_web.exe /x

• Find baseline.dat file from the extracted files and open it with notepad:
• Find ‘gencomp7788’ section and change this two entries from 1 to 0:

  InstallOnLHS=0
  InstallOnWinXP=0
  

• Save the file
• Run setup.exe /web in the extracted folder to start the installation

See also: Windows Phone 7 CTP Windows XP bypass?