Internet Explorer 501/505 Error

[chicgeek]

I'm trying to help a friend who is having trouble accessing his site for viewing. All other sites work fine, with the exception of the host's page, presumably on the same server.

Here are the details:
- two sites that produce errors are www.kawasakimotorcycle.org and www.v7inc.com
- all other users can view both sites from other locations, in both IE and FF, which indicates that it is almost certainly a problem with his system/network and not with the server.
- user is using IE, and has tried multiple systems on the same connection.
- a 501/505 error appears.

He has tried the following:
- applied 501/505 fix: returns with a DNS lookup error.
- reset modem: no change.
- pinged the website from the router/did a traceback from the router to kawasakimotorcycle.org: everything seems to be fine.

Could this be a problem with the ISP? Any one have any experience or suggestions for this?