retain Thawte certificate password in Outlook

[Esther]

Outlook 2002 SP3. I am using a Thawte certificate to digitally sign and
encrypt my messages. However my Outlook retains the password for this
certificate. While it is unlikely someone will use my computer under my
account, you cannot be too secure. I therefore want Outlook to prompt me for
the certificate password whenever I open or send a message.

How do I do this? Is it even possible?

Thanks for your help.

Esther