So I tried Google (some apps actually worked when this was happening), and I got some leads on macosxhints.com, but these tips didn't help me out. They did, however, get me thinking that it probably was a file on the Desktop that was causing the loop (I had a desktop full of stuff) ... and Path Finder was mentioned on one of the websites I had googled.
So I downloaded a Path Finder demo from cocoatech.com, and managed to open it because I had to be faster than the crashing Finder. When I finally opened it, I made a new folder on my hard drive and moved all the files I had on the Desktop to the new folder.
I did not even needed to reboot; the crashing Finder loop was over. So I don't know which file originated this, but it was finally done. It took me more than three hours to figure it out. If you have this symptom on your Mac, maybe my little horror story can help you out -- and give Path Finder a try.
[robg adds: While Path Finder is indeed worth trying (it's a previous Pick of the Week winner here), there's another somewhat more straightforward solution to this problem. Logged in as the second user, just use sudo mv in the Terminal to move all of the troublesome user's Desktop files to a new spot:
$ sudo mkdir /Users/username/Documents/tempfolder
$ sudo mv /Users/username/Desktop/* /Users/username/Documents/tempfolder
I'm sure there are other ways to do this as well, but the above would work...]