On the server, you need to ensure that the gateway is pointed to the pix inside interface or a router that uses the pix as its default gateway, that is, if you wish to continue to use the proxy as a sort of web-cache or for controlling access, etc....
Otherwise, you may want to disable auto-detection of proxy settings in the Internet Control Panel/Connections Tab/Lan Settings and/or uncheck the "Use A Proxy Server" box, and ensure that the default gateway of every client is either pointed at the pix inside interface or a router that uses the pix as its default gateway.
