Try this script:
http://www.jsifaq.com/SF/Tips/Tip.aspx?id=9651
I found it via a blog ( http://davidgardiner.blogspot.com/2007/03/preparing-for-vista-rtm-backing-up-your.html ) and it worked for me.
For interest, here's the sizes and times - showing how crazy deep the tree was (note, the one "Skipped" was the folder I was in... I accidentally issued the command from the top folder I wanted to delete, so just had to remove it afterwards):
Total Copied Skipped Mismatch FAILED Extras Dirs : 1 0 1 0 0 1541 Files : 0 0 0 0 0 25647 Bytes : 0 0 0 0 0 1.127 g Times : 0:07:35 0:00:00 0:00:00 0:07:35