I hate to even get into this subject, but here goes.
Undocumented Windows.
The problem you have is not a hardware problem. It is a software problem.
Applications running under any Microsoft Operating System except MS-DOS have a priority number.
The Compiler sets the value at a default midrange value, and most programmers do not know about this "feature" except the programmers who write OSs.
This means that there are "services" in the OS (Win2K) that have higher priority than Excel.
If a service is "hooked" (Extra subroutine called) by an inefficient program, or an inefficient program is assigned a higher priority, a lot of time is wasted.
Antivirus is one, and it was mentioned.
So, you will have to check all of the services that Win2K is running, not just the applications.
(You already checked for time piggy applications, by running a test, with Excel as the only application running.)
Next, if the problem is not in the OS services, the problem can be in the interaction problems between the Memory Manager, the Cache Manager, and Virtual Memory.
Improper placement of the TEMP Directory will aggravate the problem.
Microsoft has not documented this problem, but that is not a problem. Tools are available to check the system.
One free set is called Cacheman, and is a download from www.cnet.com (Cnet is the parent of TechRepublic.)
The Disk Cache for a regular PC should be twice the Cache RAM installed on the hard drive. Your HD probably has 8 Meg cache installed so 16Meg Max and 4 Meg Min would be the normal settings. (You can look this number up in the specifications of the hard drive. They are listed on the manufacturer's website.)
The optimum place for both the TEMP Directory and Virtual Memory, is on a separate hard drive on a separate controller. The closer you get to the Win directory, the worse.
This surprises most people, but it come from the way multithreading is implemented.
For your hardware configuration, the location of TEMP and VM is no