Hi,
First of all, without using IAS (windows 2000 Radius) , just use PIX local authentication, is that working fine or not ? If not, that means the PIX or client config still got something wrong there.
Please refer following URL for PPTP config:
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_62/config/basclnt.htm
Turn on the PPTP debug, see any error messages in the debug.
Best Regards,
Paul Qiu