Before you go out and spend money on fixing your PC, try this. Can you remember when you were last able to post with no problems. If so, have you installed anything since then, if so uninstall it, and see if it works again, then reinstall, but maybe at a lower setting if it is like Ian said a firewall or some thing like that. Also check to see if a virus has got into your system, by installing an anti virus system if you have not got one already, and do a check on your system. If you can connect to a maintenance site, like if your with BT Yahoo, or some thing like that, and get them to help to see if they can find something. It maybe a simple thing, that could cost a lot of money if you get someone else to look at it.
Hope you get it fixed, and Best Wishes.