You appear to face a host of issues with this one. Limitations of Windows XP, limitations that may be present in the drivers you are using/have been provided... etc. As I am new, I can't post all the links I needed to, so there may be links you need to select and manually copy/paste to follow.
It's not the WNIC that is at fault per se. Multiple commercial WAP devices use Mini-PCI wireless cards to provide wireless access. It appears that you need to have specific Access Point based drivers available from the manufacturer (as in the RTL 8180 Native Wi-Fi AP based devices out there), as well as an OS that can allow building an Infrastructure based device (Windows CE, for example).
Examine this MSDN article and specifically this article on configuring the registry on the OS you need to build and you start to get a sense of what would be necessary to take an OS not designed to stand as an Infrastructre WAP, and turn it into one.
As you pointed out, there are a pile of utilities that can be used in conjunction with Win7 to make it happen... however, this just wasn't the case with Windows XP. There are "potential" software solutions out there, like:
Nat32 which will provide internet access across a present WNIC (but no mention is made of it being Ad-hoc or Infrastructure).
WooWeb is a commercial software router solution that will also work across a WAN as well as a LAN, and from the documentation it appears to stand as an Infrastructure, not an Ad-Hoc.
In short: Windows XP wasn't made to be an embedded OS, which is what you essentially need to build an Infrastructure device if you don't want to use third party software. Otherwise, start downloading and testing software routers. That's your only option.