Unable to digitally sign emails yet can encrypt them in WLM

Since downloading wlm I have been unable to digitally sign my emails yet am
able to encrypt them, when I look for my imported certificates they are in
other peoples instead of in prsonal and I am unable to manually import them
into the personal folder.
In the tools/accounts/properties /security tab my certificate is shown but
when I try to send a digitally signed email I get the error message "a
problem ocurred the email was not sent" if I cancel the digital signing the
email goes OK.
Can anyone shed some light on this for me.
I have tried to repair WLM but the problem persists. I have also removed all
certificates and re-imported them and still no joy