CPU is probably not the culprit imho, but if you say CPU = 100%, check first if this is indeed the CPU and not the asio (monitor) driver who is at 100%.
If it is the asio driver that fails:
Pinning down a system who is underperforming is not easy. It can be not enough memory (you need at least 4 gig minimum), it can be buss-speed (can not reach the CPU in time), a slow harddrive (can not reach the CPU in time), and so on.
If it is in fact the CPU and nothing else (see taskmgr)
lower bitrates and sampling frequencies. The CPU can not handle the task.