HTTP Port gets automatically disabled.

Running Analog Proxy on NT Server SP6. The moment I start the proxy server HTTP port stops working. FTP and Ping still work. Even if I install someother Proxy server like wingate 5, the problem still persists.
HTTP doesn’t even work on direct connection for 10-15 minutes.
I tried changing the Prot no. for HTTP but of no use.

Could someone help pls ?