If I were a betting man though, I would almost certainly put money on it being one of those things. When XP gets installed, it defaults to having the firewall on, so unless you noticed something about that when you installed McAfee and turned it off then, it would try and run with both firewalls.
I've used McAfee in one version or another for almost 8 years now, and I've never really had any issues with it once you've got it configured how you want.
If the program is a problem though, rather than reinstalling McAfee from disk, uninstall McAfee and then go to
http://us.mcafee.com and click on my account. (that is if the internet works after uninstalling - I doubt the problem is going to arise from XP in its raw, virgin form from the fresh install) From there, sign on to your McAfee account and the first page that comes up is going to show you your subscriptions. Go ahead and download each subscription separatly starting with virus scan and do the firewall last. This way you can see if any problems arise from any individual McAfee product.