I also have same problem, I downloaded new version and installed it to iis7 and MSSQL, forum is workin and when I try to login on admin page, I get error for CAPTCHA code. I use IE8 and accept all cookies is on. Also, I didn't had any older versions instaled on server.
I try to look up for an error, and it seems that file where image is generated, CAPTCHA_image.asp don't save session of displayed image, I tested it whit reading of getSessionItem("SCS") on another page, after image was generated.
Also tryed it in a FireFox, but no luck.