I have a hard drive from an old PC and I'm trying to access the files
from it. The old PC was running Windows XP Pro and I had password
protected many of the folders.  I plugged the hard drive into my new
Windows Media Center PC. Now when I double click those folders Windows
says "Access is Denied."  I know the password for the folders, but
there's no apparent way for me to enter it. How can I access the
folder?

It's possible that I used NTFS encyrption on the disk... is there any
way to access if I know the adminstrator password?

Make sure you are logged in as Administrator.  Right click on the
folder you can't access (The one that says Access is Denied) and click
on Properties.  Now click on the Security Tab.  See if Administrator
is added in the ACL list... (It probably has some question mark stuff
or the old administrator in it)  if not click Add, type in
administrator in the box below and click ok.  Now click on full
control, Apply then OK.  This should allow you into the folder.

If it is encrypted and you have lost the keys then you can run the
recovery agent.  http://www.microsoft.com/technet/prodtechnol/winxppro/deploy/cryptfs.mspx
is a great website telling you how to do this.  Windows Media Center
is XP pro with some stuff added so it should have all the same stuff.