Appreciate it, guys.
Thanks to this post I was able to retrieve the certificate file from the old XP hard drive from this location w/o access to the OS itself:
"C:\Documents and Settings\<username>\Application Data\Microsoft\SystemCertificates\My\Certificates"
I then went to certmgr.msc and imported it into Certificates - Current User > Personal > Certificates.
But when I try to open encrypted files it still gives me access denied error, and when I try to check EFS properties it gives me this message and no way to select any certificates from the list like it says:
Any idea what am I doing wrong here?