Sounds like a CPU usage issue--possibly. When this happens do you have any other programs running, such as a browser? If so let us know what is running. If you can, call up the Task Manager (right click the Task Bar and choose it from the menu or Ctrl+Alt+Delete). Under the processes tab, see if anything is hogging CPU or memory.
If that isn't possible, tying running your computer in Safe Mode
(use F8 method only please) for a while and see if the problem still exists.
I've not heard of Shield Deluxe before and from the reviews I'm seeing at Download.com, it could well be the culprit--several reports of compatibility and performance issues. If the problem doesn't exist in safe mode, try uninstalling it and let us know if that helps.
If it does, please don't go without an antivirus for any length of time. I would suggest installing Antivir
at least temporarily as it is easy and light on resources. if you would like to explore other antivirus programs you can do so at your convenience--do not ever install more than one with active protection--and we will make some recommendations. When it comes to security programs, it is best nowdays to go with companies with good reputations.