Microsoft will be discontinuing update support for Windows XP SP1 and SP1a effective October 10, 2006.  They will then only support Windows XP SP2 unless they grant an extension.  This hasn't been publicized extensively and it's my understanding that October may be the last update. 

So folks on SP1 or SP1a may have only 2 months to move to SP2 (so that they are ready for the November updates).   SP2 works great in my corporate experiences.  However, it requires some good testing of your web apps as you have a different version of IE 6, plus you have to configure the Security Center corporately. 

You might be aware of this and I'll share this for others as well.

Scroll down to End of Support for Windows XP (note that Windows XP Embedded has to do with special Mobile computing platforms)
http://support.microsoft.com/gp/lifesupsps#Windows