There were alot of good suggestions here as to what the problem might be, however, they are guesses based on personal experiences in most cases. SVCHost is short for "Service Host" Like RunDll32, it is not possible to say what it is doing from TaskManager.
Go to Run and type CMD on NT versions/COMMAND on home versions of windows. Type (or cut and paste):
tasklist /SVC /FI "Imagename eq svchost.exe"
it will return a list similair to this:
Image Name PID Services
========================= ====== =============================================
svchost.exe 768 DcomLaunch, TermService
svchost.exe 820 RpcSs
svchost.exe 860 AudioSrv, Browser, CryptSvc, Dhcp, dmserver,
EventSystem, helpsvc, LanmanServer,
lanmanworkstation, Netman, Nla, RasMan,
Schedule, SENS, SharedAccess,
ShellHWDetection, srservice, TapiSrv,
Themes, TrkWks, W32Time, winmgmt, wscsvc,
wuauserv, WZCSVC
svchost.exe 1248 Dnscache
svchost.exe 1304 LmHosts, SSDPSRV
svchost.exe 1808 WebClient
svchost.exe 520 stisvc
There services they are running are on the right. If any of these look like something you don't want running, Type TSKILL <PROCESSID>
For example, if I wanted to kill WebClient from my list I would type
TSKILL 1248
Kill processes until you find the one that is chewing up your CPU, unfortunately, as you can see from the list, some are bundled and cannot be killed with out crippling many features (see ProcessID 860).
Good Luck