How to Fix Bad Request Error 400

1. Click the desktop's 'Start' button. Click 'All programs,' 'Accessories' and 'System Tools.' Click the 'System Restore' option. The 'System Restore' dialog will appear. This applies to both Windows XP and Vista.
2. Select the 'Restore my computer to an earlier time' option and click the 'Next' button. Select a checkpoint before the error appeared and click the 'Guided Help (Registry Backup)' option. Click the 'Next' button. Click the 'OK' button and click 'Next.' The computer will undergo the restoration process and restart. After the Windows system boots up, click the 'OK' button in the 'System Restore confirmation' window.
3. Go to the Microsoft download center Web page (see Resources) and download the Microsoft .NET Framework 1.1 Service Pack 1. Click the 'Save' button in the prompt and select a destination location for the file. Find the file and double-click the file to run it. Follow the installation wizard to install the service pack.
4. Click the desktop's 'Start' button and click the 'Run' icon. Type, without the quotes, 'Regedit' into the 'Run' text field and click the 'OK' button. The registry editor will appear.
5. Go to the left-side pane and navigate to the following entry: 'HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET.' Find 'VerificationCompatibility' in the right-side pane, right-click it and select 'Modify.' Alter the 'Value Data' to '1' and click 'OK.'