Check at Kelly's  http://www.kellys-korner-xp.com/xp_abc.htm   Select  M
and scroll down to almost end of page to
   MyDocuments-Move.

Change the Default Location of the My Documents Folder
http://support.microsoft.com/?kbid=310147

NOTE: Programs which use the CSIDL value to locate the path for a special
folder, should sense the new location of My Documents (and My Music/My
Pictures) folder. Poorly designed programs tend to default to
"%userprofile%\my documents" folder, whereas the user may have relocated the
folder already. I'd say that it's the fault of that particular application,
not Windows, as Windows provides all the API needed to locate the special
folder path programmatically. It's upto the third-party software
manufacturers to use that facility.

CSIDL:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platfor
m/shell/reference/enums/csidl.asp

<Q>
CSIDL values provide a unique system-independent way to identify special
folders used frequently by applications, but which may not have the same
name or location on any given system.
</Q>

I've installed an additional hard drive. I would like to move my existing My
Documents folder (currently on the C drive) to the new drive (D). Are there
any special precautions that I shuld take for this?

In addition, I would like those applications that now locate the My
Documents folder on the C drive to find that folder on the D drive. Are
there
settings that I can change that would enable this?

There's several ways to accomplish this. Here's one:
Right click on My Documents and select Properties. You will see a Move
button. Click it and follow the on screen directions.

Hint: I created a folder named "MyDocs" on D: before using the move button
in properties to redirect My Documents.