xp supports dual cores and hyper-threading, but not multiple cpu's.
"While Windows XP Home does not support more than one physical processor, there's a lot of confusion about whether or not it supports multiple processor cores in a single physical processor. This is implemented in several ways, of which the most widely used is Intel's Hyper-Threading technology. Also, AMD is currently rolling out a series of 64-bit processors called the X2 dual core series, which features two processor cores per die.
How much of this is supported? The answer is all of it -- in XP Professional, and, surprisingly enough, in XP Home as well. If you install XP Home on a system that has a single processor with dual cores or hyper-threading (what is referred to as two logical processors—XP Home will recognize both of them, and will install the appropriate Hardware Abstraction Layer (HAL) to work with it.
However, if you have more than one physical processor (such as two dual-core or Hyper-Threading processors), XP Home will only recognize the first one."
Does XP support multiple processor cores?
xp pro can support 2 processors, so which do you have home or pro?
Microsoft Windows Small Business Server 2003 supports 2 physical processors. These processors can be single-core, multi-core, or hyper threaded.
Please note that just because your operating system supports multiple processors it does not mean that your applications will fully utilize them. The application must be designed to support multiple processors. For more information on a specific application please contact that application's vendor.
"Microsoft Excel is not designed to be a multiprocessor-aware program; it runs on only one processor."
XL: Excel Does Not Run on Multiple Processors
These versions of vista can support 2 cpu's, the others support multi-cpu cpu's but not multiple processors. The versions are Bus N Bus Ent Ult as indicated here.
so it appears that your OS, xp 32 bit, will only support the first processor, and you should upgrade to business server OS or vista, and even then only certain app's will make use of both processors, excel unfortunately appears unable to support both according to my search, you may wish to search further....