svchost.exe (svchost stands for Service Host Process) is a Windows 2000/XP generic process which acts as a host for other processes whose proper functionning requires the use of dynamic link libraries (DLLs). There are as many entries named "svchost" as there are processes using it.
With the following command, the tlist.exe utility provided on the Windows 2000/XP CD-ROM can list all applications currently using this service:
tlist -s
The original svchost service includes a security hole, which must be fixed by updating the system using the Windows Update service.
It is not in any way a virus, a worm, a Trojan horse, spyware, or AdWare.