Http 500 error...not website owner though :S

[Archer]

Hey well im new so hello! just to get that out of the way.

Now heres a problem that will probably drive you mad because im half way their already and only been dealing with it for a day!.
im a tech but ive come across a problem thats baffling me, Im in the middle of repairing a PC (a very old PC) but I cant get to the bottom of the issue, The internet on the customers computer is functioning fine, ive done ping tests and everythings working, even their msn signs them in, but whenever they go on to a website on internet explorer they get the HTTP 500 error..... on every site :S whats going on??

I thought at first it was a virus so i done a scan but nothing was found, I updated their internet explorer and it still didnt work, I checked all drivers and couldnt find an issue, I even removed a load of start up programs incase the computer was just spazzing out due to being over worked....

I cant seem to find any information on the net about it, all the people experincing the trouble are the website owners, even my father who has been repairing computers for years has never come across this problem.

I tried to go on all known good sites and it still gave the error, i even pinged the sites too and their was no problem :S

HELP i have never come across this before.

[kmote]

Is this for all websites? What IPs do you see when you ping:
1) google
2) bbc
3) yahoo

EDIT: also what browser(s) have you tried?

[Hefemeister]

Have you repaired tcp/ip, winsock and done a dns flush via a command prompt?