Errors at startup are generally caused by programs installed at startup. The worst culprits are adware which is often badly written.
Go to Start - Run. Type in msconfid and click OK. In the dialogue box click on the Startup tab. Remove the tick from the box for any items you don't recognise and exit. If you don't get the error message at the next startup you will know that one of the unticked items was the problem.
If you have a broadband connection, you can also try an online scan with Ewido click here.