Expired web/SSL certificate error on only one computer

Answer: The Win 7 operating on the "problem computer" had had its c:\windows\system 32\drivers\etc\hosts file edited. a.contoso.com was hard-coded to point to a particular IP address (say, AAA.BBB.CCC.90). The site at AAA.BBB.CCC.90 still worked but it gave expired certificates. After creating a new site with new certificates (that oddly look the exact same as the old site) at AAA.BBB.CCC.145, the site developers must have re-directed traffic bound for a.contoso.com to the new site at AAA.BBB.CCC.145, which was serving valid certificates. I realized this by using running a trace route (tracert) on one of the working computers which resolved a.contoso.com to AAA.BBB.CCC.145. Running tracert on the "problem computer" yielded AAA.BBB.CCC.90. Browsing directly to AAA.BBB.CCC.145 on the problem computer resulted in a good site with a valid certificate! After I removed the line in host file of the "problem computer" directing traffic directly to AAA.BBB.CCC.90, everything works just fine.

Now, it seems that the problem wasn't actually with Win 7 certificates at all. The problem was with this machine directing its traffic to an old, obsolete site with old, expired certificates because the address of that site was hard-coded into its hosts file.

Thanks to everyone for helping me reach this conclusion and solve this problem.