I'm using Vista and IE7 on my primary system, and to my surprise, it complained about the certificate when going to the Paypal site. Checking the certificate, the problem wasn't with the Paypal certificate itself, but rather the VeriSign intermediate CA certificate that they were using; it was telling me that the one installed on my system had expired in January 2004.

The particularly odd thing is that I know I had been to the Paypal site earlier in the month, but it was before the previous Windows update was pushed out a couple of weeks ago. What's interesting is that this isn't an issue with Windows XP or 2003 Server. While I don't know for certain that the Windows update was the culprit, it's the last thing on my system that I can think of which could have possibly done this.

In any case, if you should run into this problem, the solution is to download the current intermediate CA certificate from VeriSign's website. Unfortunately, they strive to make this as confusing as possible by offering this labyrinth of options, so here's the link right to the certificate:

https://www.verisign.com/support/install2/intermediate.html

It will display a certificate in a textbox, so just copy it to the clipboard and save it to a text file named something like "VeriSign Intermediate CA.cer" and then you can import it by right-clicking on it and selecting Install Certificate. If you use the defaults, it installs it in the right place, which is the "Intermediate Certificate Authorities" store. Once installed, you just restart the browser and viola, life is better again and the certificate shows an expiration of 10/24/2011.