I have the exact same problem.
Uploaded a test site written in aspx and got the 500 error. Contacted Fasthost customer service and got the run around from them. They even tried replacing my web.config with their own but nothing worked. They then said that the problem was with my scripts (which they don't support) and that was how they left it. As far as they were concerned it was my problem and that was an end to their support.
So I tried something very basic: opened a new website in VWD, which it populated with the default files: default.aspx, default.aspx.vb, web.config and the App_Data folder. I simply added the a title between the <title></title> tags and a <p>Site will appear here...</p> between the <div> tags in the aspx file and uploaded all those files instead (no changes to the web.config file and no scripts in the vb file). Pretty much as basic as you can get. Guess what! Still get the 500 error.
Then I tried the following, created a default.html file with the same <title> and <p> as the aspx file above, cleared out all of the aspx stuff and uploaded the html file instead (i.e. 1 very basic html file). Now I get a 403 error!
Looked around the web and found out that Fasthosts has very unfavourable reviews on pretty much all the forums I read. Don't think I will be staying with them for very long.
Cheers.