(FWIW, That's an issue with XP Home rather than XP itself. Edit: To clarify, I mean the inability to see file and folder security permissions, not problem installing the certificate.)
You can use the cacls.exe command-line tool that's part of Windows XP, but an easier thing to do is just to creating a file in the same folder. If you can't create a file there then the folder's permissions are probably wrong.
(It can be other issues, such as a "security" tool blocking the certificate from being written. For example, Zone Alarm has caused that problem in the past.)