Currently the official XChat for Windows build is (probably) built with Visual C++ 6. It is claimed that developing the Windows version is difficult and that's why you need to pay.
However, the current source code is already more or less platform-independent (for the less part there's my patch), and for building my scripts and environment could be used. I'd be honoured if I could help in making this move.
It would also be great if the Windows and Unix source codes could finally be unified and released, and the income would be changed from payments to donations/subscriptions (PayPal, Flattr, etc.).
So dear Peter, please take these things into consideration, and make XChat a truly open-source software again.