In ASP.NET 1.1, all of the posted data is checked by the runtime to avoid the so-called HTML injection. This feature is very useful, if you want to prevent users from entering data with HTML-tags oder JavaScript. But if you want users to enter data with HTML - for example in forums - you need to disable validation of posted data. To do so, just add the following attribute to the Page-directive on your aspx-page:
ValidateRequest = "false"
You will also need to do so if you are migrating an application from ASP.NET 1.0 to 1.1.