I'm assuming you mean the Windows command prompt?
To change the current directory (like C:\DOCUME~1\LARRYD~1 , the command is cd. So if you wanted to make the current directory the root of the C drive the command would be:
What C:\DOCUME~1\LARRYD~1 basically is is My Documents. That is the default directory for the command prompt, so there's no problem.
FYI - DOCUME~1 is a shortcut for the folder Documents and Settings and LARRYD~1 is a shortcut for your username that Windows uses.