Firstly, you have to parse the whole page.
Since you use qt librarys, I recommend using the qt xml parser, which is very fast and easy to use.
After that, you have to get the captcha node. Now, you can work with it.
Depending on the captcha system, each captcha has its unique identifier, which can be posted to captcha-solving services.
You could also get the captcha picture (in most cases enclosed in some <img>...</img> - use the xml parser to find it) and show it to the user, so he/she is able to solve it.
So basically, it's not rocket science, just some simple parsing, GETting and POSTing.
It really depends on the site and captcha type, though. Also, dont expect any source code here. You should be able to do that yourself.
kind regards,
SICK