The most common cause of this is something on your desktop that is corrupt, so as soon as the Finder tries to generate the icon preview it crashes, resulting in the loop.
If you have another account on the machine, the best thing to do is log into that account, go to the problem user's Desktop folder, and move everything there into a separate folder. That will hopefully let you log into the problem user properly and sort through there to figure out what the issue is.
If you don't have another user on the machine you may still be able to create one if you can get System Preferences to open; since the Dock is probably displaying, you may be able to launch another program that way, and from there get to the Apple menu and System Prefs. You may be able to use Spotlight in the top right corner to launch an app as well--I think that isn't tied to the Finder.
You can, alternately, use a terminal window, if you can get that open (or Single User Mode) if you're familiar at all with a command line, but if not that's going to be pretty scary to deal with.
The other possibility is a corrupt Finder preferences file, which you'd address by deleting it.
This Hint has a couple of alternate suggestions:
http://www.macosxhints.com/article.php?story=20060308010111601
..and a Google search for Finder Crash Loop will give alternate tutorials.