HT is a technology present on some Intel processors that simulates the presence of more than one processor. To use this technology, the operating system must be able to support more than one CPU (Windows 9x/ME doesn’t recognize more than one CPU and doesn’t support this technology).
For example, if you have a Pentium 4 with HyperThreading technology your operating system will “think” that there are two CPUs installed, raising the system processing performance.