You will need to have Service Pack 3 installed (or Service Pack 2 if you are on 64-bit Windows XP). Also the CPU must support SSE2: http://en.wikipedia.org/wiki/SSE2. See http://audacityteam.org/download/windows#sysreq.
Also please see here about the security risks of running XP now that Microsoft no longer support it: Important information for Windows XP users.
Gale