Since you're not specifying which Windows version you're using(there are quite a few you know, and it makes a big difference), Im assuming Windows 2000?
Windows 2000 can't tell the difference between a virtual CPU and a physical one, so one HT CPU will look like two, two HT's will look like 4, etc.
You need Win2K Server to use 4 CPU's, or in this case 2 HT's.