Windows 2003 - so this is on a server right ?
There could be a number of reasons for this - it is happening all the time or intermittently ?
If it happens all the time it could be from a virus or virus removal that corrupted the file, I trust your anti-virus is up to date

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I would do a full scan of your system just to be sure then do a repair of Windows - boot with your disk in (make sure in the BIOS the DVD/CD is the first to boot from) follow the wizard until it detects your installation of Windows (NOTE:
don't choose repair console) and select it and do the repair.
That will sort out any corrupted files. BTW That method is for XP and Windows 2000 my memory is a little hazy on 2003 but you
should have that option.
EDIT: Oh just thought there may be a quicker way again memory a bit hazy on this but you could try this:
Go into the folder i386 located on c: (not the one in c:Windows) there should be a copy of the svchost.exe in there, copy and paste it into c:Windows\System32 et voila. NOTE: you will have to show hidden files and folders in explorer go to tools > folder options > view
But only do this if you sure your system is clean
