1. This error is correct. XHTML, the new standard for html, does not allow the 'width' attribute in a table cell. Instead, use something like <td style="width:50px;"></td>. This is probably happening only now because you reinstalled Visual Studio. There are settings you can change that will ignore this error, if you go into Tools->Properties, and navigate to the HTML editor section.
2. This error occurs because asp.net was uninstalled. You either need to reconfigure this site to use ASP.NET 2.0/1.x (depending on what it was to begin with), or, if the ASP.NET tab is unavailable or does not have the correct version, you need to re-register asp.net with IIS.
To re-register, open a command window (Start->Run, enter "cmd" and hit ok), and enter the following
"%systemroot%\Microsoft.NET\Framework\YOUR_VERSION\aspnet_regiis.exe" - i
(note that the -i is still typed, it is just outside the double quotes.)
Replace YOUR_VERSION with the correct version of asp.net that you want to register. Also, make sure you register them in order (if not already done so when installing visual studio), 1.0.x, 1.1.x, 2.0.x
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MCP - Web Based Client Development .NET 2.0