Long running scans can be due to several factors. The most common reason would be that there is other security software conflicting with OneCare, causing the scan to run very slowly as the scan engine and the conflicting software are both fighting over each and every file being scanned.
The other reason for a long scan would be that your PC has very large drives and many large compressed files. You can speed up the scan by excluding locations from being scanned and scanning these manually as desired. For example, if you have a 500 gig external USB drive attached and that drive is 50% full with data, the scan can take several hours. Exclude the entire drive from the scan and your total scan time should drop dramatically.
If you need help in determining if there is a conflict or how to exclude locations or files, you can contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator
