It's damn difficult if you've reinstalled and don't even have your old user profiles on the disk. According to my first link though, Microsoft and other companies will help you for a fee.
This site provides several methods for breaking the encryption. It's not easy though and not for the technically faint of heart.
There is a design flaw with EFS though which may be worth a try first - if the files are encrypted whilst not in an EFS encrypted folder then plain text copies are made temporarily that aren't encrypted. It's possible that data recovery software may pick these up, although the damn reinstall may have ruined that avenue for you as well.
The moral of the story is - export your keys! I'm shuddering to think how many people are going to get caught out by this in Vista when encryption is compulsory.
Edit:
This
Wikipedia article also covers a couple of other security flaws with EFS which would make it fairly easy to recover them if the OS was still intact. I'm just not sure whether things like the hashes can still be recovered after a reinstall though. Might be worth trying some data recovery software for things like that too.
[ 05-11-2006, 01:10 PM: Message edited by: shamrock_uk ]