Mattata,

Launch the Task Manager (Control+Alt+Del keys) and check if a process take too much CPU usage (almost 100% by example). If it’s one of SVCHOST.exe processes, download Process Explorer here http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx. With Process Explorer, you can view the services launched by SVCHOST just by moving the mouse over it. If you have an excessive CPU usage for a SVCHOST process, then I think you can suspect one of services launched by SVCHOST NETSVCS. In particular, WUAUSERV (corrupted Windows Update service?) or NETMAN (corrupted key value in Windows Register?). If you have encountered this CPU usage problem, it’s easy to check if it’s a fault of a service launched by Windows at booting.

Run the command CMD or open the console and type these commands:
NET STOP wuauserv (to stop the Windows Update Service, to see if CPU came back to normal usage)

If the problem of slow PC still exist, then type the following command:
NET STOP netman (to stop Network Connections Service, to see if CPU came back to normal usage)

Don’t panic, the services will be restarted after reboot or you can use the commands NET START wuauserv or NET START netman.
If stopping one of the two services have solved the problem (until reboot) then there is a solution for your computer.