I've never actually heard of a stall on that step. I typically disable that option anyway as I tend to empty temp and temporary Internet files when I feel like it. If you want to pursue this, I'd suggest first trying that option manually - it is found at Start/All programs/Accessories/System Tools/Disc Cleanup.
If it runs fine there, but not from within OneCare Tune-up, contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
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