First of all you should absolutely consider upgrading your SQL Server instance. If you can afford to migrate to SQL Server 2005 SP2 there are loads of performance and administration enhancements you can take advantage of; if you want to (or need to) stay with SQL Server 2000 you should seriously consider upgrading to SP4 (especially to protect yourself from some very well publicised security vulnerabilities).

As for how SQL Server utilizes resources, I would definitely need more information to provide any definitive advice. What does your application workload look like? Do you have a large number of concurrent connections? What type of an application are you running? Are there a large number of calculations being performed in the database? You should create a performance baseline using PerfMon counters to determine the normal operating behavior for your application. Next, you can read articles like Optimizing SQL Server CPU Performance to determine whether some basic tuning can help, or if you need more detailed advice.