The best method will depend on your personal setup,
but here are the basic steps -
1.Copy the "Windows Easy Transfer"(WET) software from your Vista PC to your XP PC.
2.Run WET on the XP PC. - follow the wizard instructions.
3.Run WET on the Vista PC - follow the wizard.
The main decision you have to make is,which transfer method.
CD/DVD - USB - slow but straight forward
External HD - requires you have External HD
Easy Transfer Cable - Fast,easy,but it costs $50 - 60
HERE
Via Network - both PCs have to be Networked
Please be aware that you can`t transfer programs,just program data.
You will have to install Vista versions of all your programs,onto the Vista PC.
Further reading
HERE and
HERE