There are many reasons why a PC fan will run alot and be noisy. It could be due to the fact that the vents are clogged and there isn't enough air flow to cool the inside of the PC. This is more common in laptops.
However, since you mention a 10 hour long scan, I think that you are running more than one security product at the same time that are providing "real time" protection. This will cause the condition you describe - a long running scan and the processor running hot because it is overworked. When two security programs are fighting over access to every file, process, and disc access, the PC never gets a chance to rest.
Make sure that you only have one antivirus, antispyware, and firewall in place. If you are using Windows Live OneCare, it should be the only installed, running security product.
If you need help in removing the other conflicting software or are not sure what is causing the problem, you can reach OneCare support per the instructions here:
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
Additionally, you already had a thread on this problem in another forum topic folder. I answered that earlier today with some questions for you.
http://social.microsoft.com/Forums/en-US/onecareanti-virus/thread/8513c433-afbc-4ba6-b44c-1ca777900c43
Please continue that thread instead of this one.
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator
