Ok, that is your problem.
What I strongly suggest you do, and this is also what LANDesk suggests...
Only select Vulnerabilities, LANDesk Updates and Custom Definitions.
If you are using AntiVirus from LANDesk, you will want to keep that one checked.
Spyware and Blocked Applications are HUGE CPU Hogs and take a long time to complete.
What you can do for those is, create a different scan setting, and then create a new scheduled task and schedule it to run once a month, or more often if you wish.
Make sure that all of the other settings, especially the reboot options in this new Scan setting are set properly! It might be best to right click the existing one and Copy it, rename it, and make the needed changes.
I can guarantee you that these changes will make a huge difference of your machines. We have a bunch of old junky machines and have no issues