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Dllhost.exe is just a wrapper that loads and runs a Dll as if it were an exe. To know what is going on you need to look in task manager to see what Dlls are being run and why.
Edit: On my HP desktops if the drive was so busy I couldn't use it, usually it was Windows Media Player trying to find every media file on my system to share it over the network. Look in task manager for mobsync.exe. If it is running that may be the issue. But it can be drive indexing or any number of things.