Let me tell you what it is not: On Windows XP, 2000 and 2003, svchost is not a virus. On those systems svchost is a required system component. If you happen to successfully delete it, your system will not run. You'll be much worse off than before. (Win95, 98 and Me users, see Note 1.)
Don't delete svchost.exe. Don't even think about it. [Important: do not confuse svchost, which we are discussing here, with scvhost, which has two letters transposed. They are not the same thing. The presence of scvhost may indicate a virus.]
Svchost, which is short for "service host", is a core part of the operating system that provides support to many of the required services that are Windows. You can see all the copies of svchost and what services they are running by typing "tasklist /svc" in a command window. If you don't have tasklist, or just prefer not to use the command shell, you can use SysInternals Process Explorer instead. (Check out my previous article "What is Tasklist.exe, and why don't I have it?" for details.) On my machine one copy of svchost is responsible for 30 separate services, another is hosting 4, and the remaining 3 host one service apiece.
