Is the posgres service running? Open Task Manager > Processes tab. Do you see any postgres processes?
Try disabling your AV software again - it may well be preventing postgres starting.
If postgres still won't start (Start > Programs > Postgres > Start Service) please zip (select; right click > Send to Compressed Folder) the last few files in the C:\Program Files\PostgreSQL\8.3\data\pg_log folder to a
Support ticket.