Differences between win7 64bit and win xp

Hi, I have two computers. One running windows7 64bit, and the other running windows xp home.  Both are running the same version of Norton Internet Security. If I go to  www.Hobbycomponents.com on my xp machine it comes up with certificate invalid, and shows the whole top bar in red.   The win7 machine says the site is ok!.  How can I tell which one is accurate, as I do a fair bit of trade with this company, and I wouldnt want to offend them by telling them there seems to be a problem with their websites security certificate.

regards  john