It started with being unable to access hotmail. Neither Firefox 3 nor IE7 would load. It would timeout and act as if I had no connection. As far as I know, my firewall isn't blocking them (Zonealarm Security Sweet, newest version) and my host file is fine. (Only one entry, local host, 127.0.0.1, I think.) Clearing the cache and cookies did nothing.
I ran many scans with ZA antivirus/spyware, also SUPERantispyware, Malwarebytes Antimalware, and a Symantec online scan. A lot of stuff got picked up and removed, including some Vundo I never knew I had. Repeated recent scans seem fine. (Oh, and Microsoft Malicious Software Removal tool was run, too.)
I also found I couldn't run Windows update. I'd get errors that were consistent with being unable to connect to the site, and after trying some fixes based on a Knowledge Base article (that involve messing with Cryptographic Services and renaming Catroot2 folder) I'd get partially able to run updates, and got as far as being able to sorta install SP3, but it'd crap out on me and give me errors which I could not get details for through the site....
Incidentally, I also can't access geekstogo.com. I'm able to connect to all the sites that I can't on other systems and other networks, so I know it's just the one PC, and not that the sites are down. Almost all other web-browsing and internet activities work.
Where should I start to troubleshoot this?