I originally marked your post as off topic and was about to move it, when I saw the reference to msmpeng.exe
Are you using Windows Live OneCare? Or are you running Windows Defender. If only the latter, you are off topic for this forum. If the former, you are on topic, though I'm not sure why you are getting the error. Basically, newdotnet is likely to be spyware. If OneCare or Defender are reporting an error, it would seem that removal of the spyware was incomplete.
If you are using Windows Live OneCare and you have been infected, but OneCare did not detect or cannot remove the malware, please contact support to report this and for help with removal.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If you are in North America, you can call 866-727-2338 for help with virus and spyware infections. See http://www.microsoft.com/protect/support/default.mspx for details. For international information, see your local subsidiary Support site.
-steve