c:\documents and settings\youroldusername
When you created a new OS and logged on, even if you used the same user name, Windows looked on the hard drive and saw that there was already an unknown user profile stored there, so it put your user profile in a different directory.
Next time, make backups before doing something like that!
(And you're not going to have read permissions to your old profile under the new OS. You'll have to take ownership of the files there and give yourself read permissions.)