Preconfigured Sharepoint 2007 installation from Microsoft:
http://www.microsoft.com/downloads/details.aspx?FamilyID=67f93dcb-ada8-4db5-a47b-df17e14b2c74&displaylang=en
Some great tips regarding Sharepoint dev environment setup:
http://www.sharepointdevwiki.com/display/public/Building+a+SharePoint+Development+Environment
Update 19/08/2010:
SharePoint 2010
http://www.sharepointbuzz.com/archive/2010/05/11/sharepoint-2010-virtual-machine-rtm-download-links.aspx
