Outgoing on port 80/http blocked - how to get rid of blocking??

All other access to the Internet is working on all other protocols from
this maching but not port 80. Even port 81 works through IE/Firefox etc
but some malware I suppose has changed some config param somewhere to
redirect/block outgoing trafic on port 80. Where can this have been set
in the system and how can it be undone?

Any hints appreciated

TIA


BoB