Your using Windows Vista and it's known to drain PC resources. Anyways, can you please post the resource monitor when mcshield was taking up all the CPU resources?
Two quick notes : first, your processor is 1.6GHz, which is a bit on the slow side for intensive real-time processing (which is what mcshield is doing). If you can get a compatible upgrade, that would help. And second, mcshield is checking files being accessed by other programs, so look to see what programs and services are running during the start-up process and keep those down to the minimum.
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