Yeah, I agree. You probably won't find a fully HTML validated VBulletin site. But 277 errors is quite a lot and a lot of your errors are just based around it not having an alt tag specified. You could reduce the amount of errors.
If you manage to figure out why the forum became blocked by IE7PRO on that one page, then please could you PM me and let me know why it was. I'm a website designer and use a lot of ecommerce and bulletin scripts. So It might be quite interesting to know.
Cheers Anthony!