Make sure it isn't a fake svchost or a malicious service first. Search on C:/ for "svchost" (with Advanced Options ticked, so you can search for hidden/system files) and make sure there is only ONE copy on the drive, which should be located in "C:/WINDOWS/system32" or "C:/WINNT/system32".
"svchost" is a host process for apps that run as services, so it's is perfectly normal to have multiple copies running at once as there are a number of services that come with Windows that run under svchost.Look for any services that look suspicious, don't have anything todo with programs you've installed or drivers you are using, or that don't have the Manafacturer field as "Microsoft Corporation" in the Services tab in MSConfig. Having multiple copies of "svchost" running is TOTALLY NORMAL, I repeat, TOTALLY NORMAL, as many services use it as a host, hence why it is called "service host", however there are alot of trojans and viruses out there that masquerade as legitimate services that use svchost.
I've seen one virus that created 2 services which were flagged as "essential", and when one of them was disabled/removed/stopped the other one would recreate it, so be aware of what the usual processes that run under Windows are.
And don't go stopping random copies of "svchost" from Task Manager, as you aren't doing anything except from stopping services. If you stop the copy that is hosting the Remote Procedure Call, you'll get a cute message telling you that the RPC has crashed and your box will restart after a number of seconds (usually half a minute), this is normal behaviour too, as the RPC is a vital part of WinNT. If you stop the copy of "svchost" that is hosting "Windows Audio", your audio will stop working as this is a layer between the sound drivers and the kernel (if I remember).
As for your memory-hogging "svchost", you most likely have a malicious service, or a corrupted service being hosted under it. You should try disabling all but the necessary "svchost" services in services.msc (you can see which ones use it in their properties page) and reenabling them until you find out which one keeps bugging you, when you know which service is causing the problem, tell me which.