You need to check the task manager when the issue is happening to find the bottleneck.
You can also use the performance counters to do this automatically. Once you have the data you will see if it is a hard drive issue, processor or memory. Is your Database on the same drive as the operating system? What is the speed of the disk?
Make sure your data and os are separate. Add more memory if possible and see if that corrects your issue.
A great article for learning about performance counters and this stuff is here:
Feel free to email me with your results and I will help you break it down. karl at asktheadmin dot com.