Digital Signature

I have installed a digital certificate in outlook from Thawte on a user's
computer. When I tried to send his signature to others, it wouldn't let me.
The error "Your digital ID name can not be found by the underlying security
system" came up. When I went to Tools>Options>Security and click on
Settings, MD5 and RC2-128 bit are selected by default. Every other user uses
SHA1 and 3DES, but these options aren't even available for this user. I
can't find any info on this problem, and I thought someone here might help.
I've got Windows XP Pro and Office Pro 2003.

Thank you in advance,

Adam