I'll try it now...the second I figure out what the config file is. :-)
Are you running the GUI or the console?
If GUI, you will have a pretty red flower in the lower right corner. Right-click on the flower and up will pop a drop down list. Choose Configure and you will find your user name in a box. Edit (don't misspel) and save. Restart the computer.
If console: The easiest way is to reinstall the application, entering pbrown and 36120 at the start. Go through the advanced setup options and complete all the options as before.
You can also go to My Computer/Program Files/Folding@Home/config.txt. Use a text editor like Notepad, and only change what needs to be changed.