There will be several instances of svchost running.  It's a generic service that runs when some other application, service or even hardware is running.

YOU NEED TO FIND OUT WHAT IS CAUSING SVCHOST TO USE SO MUCH CPU CYCLES.

Here's how. Microsoft has a free tool you can download, called Process Explorer. It's sort of like Task Manager but when you hover over each instance of SVCHOST, it will show you the associated tasks tied to each one. One innocent SVCHOST could be your modem. The "bad" one may be some software trying to check for updates via the internet. You could then disable that software and try to reinstall it.

it's NOT SVCHOST that is the problem. It's the underlying software, service, or even hardware that is causing that instance of SVCHOST to run so much.

Do a google search for Process Explorer. It's free from Microsoft. Download it and run it. See which SVCHOST is running the most and what is causing it.

How to stop svchost.exe using up 100% system resources (Windows XP Only): 

• Visit the Microsoft website and Download Windows Update v3 WindowsUpdateAgent30-x86.exe and save it to your C:\ drive
• Download this file WindowsXP-KB927891.exe (right click and choose save as..) and save it to your C:\ drive
• Run WindowsUpdateAgent30-x86.exe
• When Windows Update Agent finishes installing, run WindowsXP-KB927891.exe and follow the prompts as it installs.
• Reboot the computer