Browser not loading some websites

Hi All
I formatted and reloaded a friend's PC last week. I did a clean load of XP SP3 with WMP11 (I used nLite to make a slip-streamed XP install CD - worked a treat! :D). The list of progs installed: Avast Home, FF 3.0.3, IE 6, Foxit Reader, Office 2007 Std Academic (my friend is a teacher), SpyBot, CCleaner, Advanced WindowsCare V2 Personal, JKDefrag, CD Burner XPPro, Picasa, Windows Messenger 2009, OE (just so he has access to all his old emails even tho he's swapped to Outlook), Google Earth. All are the latest versions, with Messenger & Picasa being BETAs. There were no glitches during install, and I did my usual array of tweaking and registry optimization using Registry Mechanic, TuneUp Utilities and Advanced WindowsCare (the same way I do on all my clean installs). The weird thing is that once the PC was up and finished, I can surf to some websites, not others. It is not dependent on country or type (e.g. some .orgs work, some don't), and has the same result in either IE or FF. I tried his new Vista PC off the same ADSL modem with the same settings using the same cable - no problems. Interestingly, restarting the PC yields EXACTLY the same results: that is, the same sites that wouldn't load before don't load now. The error message is one of 2: either the Google search page comes up showing similar sites (and the site in question is listed in the results: clicking on the result of the search on the desired / faulty site still yields no load or a loop back to the search page), or the website not found error page.

I have 3 theories:
1) It has something to do with Office 2007. I don't trust that app as far as I can spit it, esp on XP.
2) SpyBot or Advanced WindowsCare has messed up the blocked sites section of IE & FF.
3) One of the registry cleaning / optimization operations has crapped something out. Note that I use the default optimization settings for Registry Mechanics "recommended" Tune Up Your Services, TuneUp Utilities 2008 System Optimizer (both System and Internet), and Advanced WindowsCare "system optimization". These have never caused problems to date.

I don't really want to do a reformat and install, as I don't know what the problem is or if I will end up in the same place again. I've never come across this before, and am following the normal "procedure" I have developed over the last 5 years for clean installs without (known) variation. Has anyone got any ideas as to where I can start looking to figure this problem out? Clearly, my friend can still use his Vista PC to surf to get "around" the problem, but that's not the point. I've done a Google for the problem but have not found one that matches these symptoms exactly.

All suggestions very welcome! :o

Cheers
Peter

To check if it's spybot, hit the IE tools menu /spybot entry. When my browser craps out, I run x-setup's clean up code pages plugin. Try doing a thorough virus/trojan/worm/general slimy thing scan. Check the HOSTS file. In IE, check the Restricted sites. Search the registry for any reference to a site that doesn't work.

I'm running Vista 32-bit, with both Office 2007 and IE 8 beta, and haven't run into this problem. I also use Avast (paid version), ccleaner, jkdefrag. Check Windows Defender and Firewall settings. And there's an MSKB article on nLite.

In other words, throw everything at the wall and see what sticks.