I just started using sitefinity a couple of weeks ago. I downloaded the blank project and created a test website for 3.6 originally and then today I downloaded 3.7 and started from scratch again. I've setup SQL Server 2005 and IIS successfully (including adding the three extension mappings to aspnet_isapi.dll) and can administer the website.
Unfortunately, both 3.6 and 3.7 have exhibited the same nasty behaviour in IE 7. The RadEditor hangs when it tries to load for a Generic Content control anywhere in the admin site (in a template a Page or from the Generic Content Module). A blank white pane is displayed but none of the toolbars show up and the page shows as perpetually loading. In that state, I cannot enter any text. I have to halt the page from loading which does cause the toolboars to show up. Sometimes things work Ok after this and I can enter text and save the info. Other times sitefinity becomes unresponsive and I have to close the website complete. Occasionally, this breaks IIS completely wherein I get a "There are too many people accessing the Web site at this time." error when I try to relaunch it. I then have to reset IIS in order to be able to get back into sitefinity.
I've cleared all the IE history, temp files, cookies, etc. but that didn't help.
I'm running Windows XP SP3 and IE7. Curiously, I do not see this behaviour in Firefox 3.5.2 so maybe it's an IE setting somewhere that is the culprit.