Here was a frustrating situation: After setting up a key for my dev box, I couldn’t connect.  Putty and TortoiseSVN would die on an error of too many authentication failures.  There was no additional information on the server other than this error, and, if I connected from another system, things worked fine.

The secret here: I had six active keys in pageant.  I removed all but the key for my dev box, and now it works like a top.