WordPress 404's

Hey all. I'm running Zen Cart 1.3.7, and I've used the WordPress contribution for the site I'm working on. It's working - when you come to the initial WordPress page. But if you click on the title or the comments link (to get to the single post page), or that archives - or really any link that has to do with WordPress, I get Zen Cart's 404 page.

As found in another thread, if I change the "?" to "&", then the page comes up just fine, and the links work. But this is odd to me - it doesn't seem like it should be doing this.

In the other thread, no solution was found. The last person who had this issue said they would try uninstalling/reinstalling - but never came back. I tried the same, and I'm having the same problem still - so it didn't work.

I must have this working - I can't install it as a separate thing outside of Zen Cart, that's not an option, otherwise I'd've already done it - would anyone have any ideas for me on how to fix this?