XP, Internet Explorer, and Certificate Errors

Hello. I'm very concerned about the following problem. Please help if you can.

Internet Explorer gives me a certificate error message for EVERY security certificate-based site.

It appears as follows: "Certificate Error: Navigation Blocked"

"There is a problem with this website's security certificate. 
 The security certificate presented by this website was not issued by a trusted certificate authority.

Security certificate problems may indicate an attempt to fool you or intercept any data you send to the server. 
  We recommend that you close this webpage and do not continue to this website. 
  Click here to close this webpage. 
  Continue to this website (not recommended). 
     More information"

This has happened on sites I've previously visited hundreds of time without incident (my e-mail, bank, etc.).

Sites with site key images do not show the pictures.

I'm especially concerned about the " intercept any data" part of the warning, but I do know that the legitimate sites have received my information when I've chosen to continue on to the website anyway.

I've been having this problem since I made three modifications at the end of last week:

1. I installed Windows XP service pack 3

2. I installed a new Root Certificates Update

3. I began service with a new provider (Comcast - and began using their new modem)

A virus scan and windows defender scan came up clean.

Should I post a log in the Hijack This forum? Is there something else I can do?

My specs:

Dell Dimension E310
3.0 GHz processor
1 GB RAM

Windows XP  5.1.2600 Service Pack 3, Build 2600

IE 7.0.5730.11
Kaspersky AntiVirus and Internet Security 7.0.0125d.ff
Windows Defender
I've made all updates through Microsoft Update or Automatic Update.

(My many previous Root Certficate updates caused no problems).

I'd appreciate any help anyone can offer.