Stubborn Certificate Error

Hi, here's the scenario: Computer= XP Pro SP3 IE 8, Outlook 2007 SP3
Migrated bunch of users in an office from pop3 mail to a resold Cloud Exchange service. All went apart from one (there's always one, my o my is there always one!). I won't involve outlook at this time cos the problem crops up in IE 8 when trying to access the OWA site. Basically we get the certificated issued was not by a trusted third party authority. (It clearly is, as all other machines work and this has never cropped up before, there is NO doubting the certificate). I have removed any instances of the cert from the mmc console (from both user account and local computer lists) and it doesn't appear in the untrusted lists anywhere. Installed the cert in the trusted certification authorities and despite all this IE refuses to trust the issuer. This problem shouldn't exist as we've done hundreds of these migrations and never come across this situation before. Tried logging on as a different user - same issue. The owa site is . The date and time on the PC is correct and they don't get this problem going to other ssl cert sites. So... how do I reset the situation to scratch? how can I clear out the cert stores or cleanse them somehow?  or is there something else at play here?

Thanks Experts
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RealtecComputingConnect With a Mentor Author Commented:
Thanks for the comment. I found the answer after much searching. I simply removed all instances of the cert from the stores then went to a working computer and found two certs to do with this issuer and exported both certs from the trusted root cert auth store to a usb stick then imported them to the broken PC.
RobOwner (Aidellio)Commented:
Is this machine up to do with Windows Updates?  Especially the root certificate updates as these contain the CA (Certified Authorities) that IE uses to validate against
RealtecComputingAuthor Commented:
Reason I accepted my own solution was because it fixed the problem.
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