Moved from Security Awareness to Business section, VirusScan Enterprise, for attention.
This sounds almost exactly like an issue we had at my location a while back. After investigation we determined that the method in which ScriptScan inspects all scripts was not condusive to our business needs. Once we disabled it we had no further issues.
Policy Catalog > VirusScan Enterprise 8.7/8.8 > On-Access General Policies > Your Default Policy > ScriptScan Tab > Remove the check
I use XP Pro and solved the problem of mschield excessive cpu usage is by changing the system properties, Performance Options as follows:
Adjust for best performance of
Processor scheduling : programs
Memory usage: System Cache