The version of your ASP.NET framework installation is irrelevant -
you can run previous versions of the toolkit on newer versions of the
framework quite happily.
This is the page you need for running in vs2005:
http://ajaxcontroltoolkit.codeplex.com/Release/ProjectReleases.aspx?ReleaseId=11121
IMPORTANT: Make sure to get the Ajax Extensions v1.0 from here: http://www.asp.net/ajax/downloads/
The top link (AjaxControlToolkit.zip)
should suffice, this is however no longer supported. Unfortunately, you
will not be able to run the latest version of the toolkit, but version
20229 is pretty stable anyway. Bear in mind that this version is
targetted for ASP.NET 2.0
Is there any reason why you can't use Visual Web Developer 2008?
That will allow you to use the latest version of the framework. You can
use this software for commercial use also, and it is completely free. It acts as a nice stopgap until you are able to buy the full package, it's certainly not as crippled as other people make out.
Let me know if you need any futher information.
Thanks,
Steve