Encrypting files on XP

Why is that when I right click on a file and select advanced the check box to encrypt file can't be checked. My C: drive is already in NTFS format so that's not it. PLEASE can someone help me!

XP home or XP pro? I think that might have a bearing on the options available

home

http://www.iopus.com/guides/efs.htm

apparently nothing says that home can do it. only pro.. 🙁
maybe you can find someone that knows something about home edition

I have 2 computers, a laptop and a desktop. Both run windows XP Home. Within one month I had several problems on these systems and had to reinstall the operating systems. PRIOR to the reinstallations, both computers were ABLE to use microsofts encryption. I used it often on both computers. When I reinstalled, both computers are UNABLE to utilize this function. Could it be the updates that caused it to work? I know microsoft has added many security updates.