http://www.technibble.com/how-to-fix-svchost-using-100-cpu-memory-leak/
below taken from above article:
Visit the Microsoft website and Download Windows Update v3 WindowsUpdateAgent30-x86.exe and save it to your C:\ drive
Download this file fix_svchost.bat (right click and choose save as..) and save it to your C:\ drive
Download this file WindowsXP-KB927891.exe and save it to your C:\ drive
Reboot the computer and log in to Windows XP in safe mode. To do this, press F8 just before the WindowsXP logo shows up during boot and press up to choose “Safe Mode”
Once Windows has loaded and you have the option of which user account to use, log on as “Administrator”.
Click Start > Run, choose the Browse button and find the fix_svchost.bat file you saved before, press Open, then OK.
A black screen will pop up and white text will scroll past. Wait for this process to finish as it could take several minutes. It will close itself when its finished.
Once the black screen disappears, Click Start > Run, choose the Browse button and find the WindowsUpdateAgent30-x86.exe file you saved before, press Open, then OK. Follow the prompts as it installs.
When Windows Update Agent finishes installing, Click Start > Run, choose the Browse button and find the WindowsXP-KB927891.exe file you saved before, press Open, then OK. Follow the prompts as it installs.
Reboot the computer
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you can also download Autoruns:
http://technet.microsoft.com/en-us/sysinternals/bb963902
I would scrutinize what you have loading on StartUp. You need to see if you have programs loading during boot process which might be changed to on demand.
With Autoruns you can uncheck an item, which disables it from starting, or you can right click an item and then delete it. If you uncheck you can recheck to re-enable the item. It is a much safer approach than editing the Registry and better than using msconfig.
Another useful feature of the program is that you can right click an item and select Search Online to get information about the item selected.
When using Autoruns you will need to click on the "Logon" tab to see what is running at start-up.
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and Process explorer which is a more robust task manager, you can search each item on line and find out what it is:
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Process Explorer
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some info on autoruns:
http://www.bleepingcomputer.com/forums/t/33012/how-to-use-the-startup-database/
Bleeping computer on Autoruns