Buffer Under Runs

I'm having a problem with buffer under runs with Home Studio 7.
 
My OS is 32 bit Windows Vista. My CPU is a Core 2 Duo. I have 4 GB of RAM. Windows Task Manager indicates that there is no correlation between the dropouts and amount of CPU usage.
 
I have increased the size of the audio buffers from the default of 150 milliseconds to the maximum of 340 milliseconds. The increase in buffer size has reduced the frequency of the dropouts, but has not eliminated them entirely.
 
I am currently using 4 audio buffers, which is the default for Home Studio 7. What is the maximum number of audio buffers that my computer can accommodate?