Silly question to start. Is the most recent SP installed for 6.5? If so, move on to the rest of the answer.
There is an option that a lot of people set call priority boost (I think it was called then) that raises the priority of the SQL thread, I have generally had bad experiences with this. Such as when the server gets busy it won't respond to SQL login packets..
Now with that said, this might be normal, SQL 6.5, 7, 2000, 2005, 2008, can and WILL use all resources available to it. This is especially true on a single processor(1 core) box. You should set the max SQL memory to save at least 256MB with 2GB of hard memory. More if you have middleware or other services on the box.
Not sure if I helped or clouded the issue, let me know..