Could not load file or assembly "Microsoft.Web.Extensions"

I've inherited an asp.net 3.5 solution and I'm having trouble getting it working. I keep receiving the error:

Could not load file or assembly 'Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified. (c:\mydir\web.config line 54)

Now, I've had a good look around Google and have seen that people have said to run a simple search and replace to change "Microsoft.Web." to "System.Web." as the namespaces have changed, but I'm having no luck at all.

These are the pertinent lines of the Web.config file

<pages validateRequest="True">
    <controls>
        <add tagPrefix="asp" namespace="Microsoft.Web.UI" assembly="Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
            <add tagPrefix="asp" namespace="Microsoft.Web.UI.Controls" assembly="Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    </controls>
    <tagMapping>
        Line 54: <add tagType="System.Web.UI.WebControls.CompareValidator" mappedTagType="Microsoft.Web.UI.Compatibility.CompareValidator, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        <add tagType="System.Web.UI.WebControls.CustomValidator" mappedTagType="Microsoft.Web.UI.Compatibility.CustomValidator, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        <add tagType="System.Web.UI.WebControls.RangeValidator" mappedTagType="Microsoft.Web.UI.Compatibility.RangeValidator, Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        <add tagType="System.Web.UI.WebControls.RegularExpressionValidator" mappedTagType="Microsoft.Web.UI.Compatibility.RegularExpre