I was having the exact same problem so I reinstalled IIS, ran aspnet_regiis.exe -i and reinstalled lansweeper. Now I'm getting a different error:
Server Error in '/lansweeper' Application.
Configuration Error
Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.
Parser Error Message: A procedure imported by 'System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' could not be loaded.
Source Error:
Line 8: </appSettings> Line 9: <system.web> Line 10: <compilation debug="false" strict="false" explicit="true"> Line 11: <assemblies><add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/> Line 12: <add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
Source File: C:\Program Files\lansweeper\website\web.config Line: 10
Version Information: Microsoft .NET Framework Version:2.0.50215.44; ASP.NET Version:2.0.50215.44