Is this only happening during on-demand scans? If so, check that you have the client task set to the lowest possible utilization setting. If it is from on-access scanning, you may need to look at your exclusions.
Also, there is a common misconception with resource utilization. If you are not familiar, please read about Process Scheduling.
You will likely see McShield.exe running at "Normal" and Scan64.exe running at "Low" priority. Most applications run at a higher value than Scan64.exe, so if there is competition for CPU cycles, your other applications will win. However, this isn't perfect, as the Scan64.exe process is only feeding a list of targets for McShield.exe to munch on, and McShield.exe is probably on equal footing with your other applications.
It may also help to consider what you are scanning. If you are hitting compressed files, then they will be expanded everytime they are scanned--adding tremendous overhead. Sometimes making a slight change to the scanning strategy makes a huge difference. I've seen people over-complicate this, but it is worth at least a few minutes of discussion.
Good Information Joe