Windows XP final release

"The only available trial that I know of is the Windows 7 Release Candidate, keeping in mind that:

Downloads will be available until August 20, 2009
The RC will expire on June 1, 2010.

There are not trials for XP or for Vista."

"The normal procedure would be:

install Windows XP from your installation medium (CD-ROM for example)
apply the latest service pack (currently that's Service Pack 3)
run Windows Update and apply all patches

Regarding patching/updating Windows: there is a nice solution if you have to do this for multiple computers and want to minimize your download time and volume. heise security, a website of a German magazine publisher, has a nice, free solution for packaging Windows Updates:

DIY Service Pack - Installing Windows updates without an internet connection

Looking for manageable Windows updates
even without an internet connection?
Our offline update 3.0 script
collection downloads the entire body
of updates for Windows 2000, XP or
Server 2003 from Microsoft's servers
in one fell swoop and then uses them
to create patch packages on CD, DVD or
USB stick. Those in turn allow you to
update as many PCs as desired."

If you're willing to do the slipstreaming of SP3, you can also use this Elder Geek's guide.

"You determine the final release by looking at the roadmap. It also links you to the service pack to download (SP3). Since you cannot buy it in stores anymore, you must copy someone else's disk. If the disk does not include SP3, you must go through a slipstreaming dance. Once you have this Win XP with SP3 disk you can use it to install on the intended machine provided you have an activation key. You can still buy activation keys by going through the Microsoft Genuine Advantage thing, or by calling them, if that's what you actually need. They're not all talk, they have an accounts receivable department.

If you are in college, inquire with you're college first. They might have paid the Microsoft tax as part of your tuition which entitles you to certain things, possibly a license to windows and office. Probably free versions of Visual Studio and other products.

Now, although most stores you walk into don't carry it anymore, you can get it online. Such as from NewEgg."