On April 21st, 2008... it was announced that 3rd refreshment to Windows XP in form of Service Pack was completed. Stay put for now, since it requires some time to post it to the web (Windows Update, etc.). But the work has been finished. I personally tested it already and I assure you - installation is piece of cake.
What is there for me? - you ask. There is no better place to get to know that than Windows XP Serive Pack 3 Overview. In short: over one thousand bug fixes, exciting features known from Vista (e.g. Network Access Protection), better cryptographic modules in kernel, black hole router detection.
It will be available for download since April 29th, 2008.