I found this from click here
"Throughout the beta we've seen that certain installs of Microsoft
>Antispyware involve high CPU loads and significant impact on both individual
>apps and the general use of the machine.
>I'd recommend trying this set of steps to see whether it provides relief on
>a given machine.
>Beta2 will be quite different, I believe, but it won't be available before
>year end, if then.
>1) uninstall Microsoft Antispyware
>2) delete \program files\microsoft antispyware and all subfolders
>You will lose access to all quarantined items, history, and custom settings
>3) reinstall Microsoft Antispyware.
>This may fix a variety of ills. One easy thing you could check is whether
>the errors.log file in the \program files\microsoft antispyware folder is
>large. If so, delete it. Growth under earlier builds was uncapped and
>could be rapid--2 gig files under NTFS were possible. Growth of this file
>is now capped, so simply deleting it will fix that issue."
If you download and install the latest version you shouldn't get the same problem.