Windows Update service causes high processor usage
Microsoft released what they called a “fix” to prevent this very thing from happening but it didn’t take on some systems and the problem persisted. The problem revolves around what Microsoft calls the “svchost/msi” issue in which svchost.exe and its various processes will use 100% of the processor during update scans or downloads.
The simple solution for this is to simply turn off automatic updates on Windows XP and 2003 systems; Vista does not appear to be affected by this flaw (for once). If you don’t want to deactivate Windows Update then you might try using
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