The quickest way to get rid of infections and repair the installation on an OEM machine would be to back up the personal stuff and factory reset, which may still be the best option given that some things will only work in Safe Mode.
As for the high CPU usage, I've never used Vista but would think it similar to Win 7.
Are all of your Windows Updates up to date as it could be the Windows Update service that's hogging.
You could check that by going Start - type services.msc and press enter then scroll down to the Windows Update service, click on it then on Stop in the upper left pane then check the CPU usage.
Also, right click on the task bar and select Task Manager and under the Processes tab click on Memory to sort to the highest user first.
Right click on the highest user and select Go to services and the one that is highlighted will be the hog.
Which antivirus program is installed on the computer ?
Go Start - type msconfig and press enter then under the Startup tab click on Disable all - Apply - OK - Restart then check the CPU usage.
If that brings it down and your antivirus program isn't among those disabled then you can leave all of those items disabled, unless you want to find which is the culprit.
You would then enable one at a time - Apply - OK - Restart and check the CPU after each.
If it is still maxed out then under the Sevices tab check the box to Hide all Microsoft services then Disable all - Apply - OK - Restart
If that brings the CPU down then add a couple of the services back at a time - Apply - OK - Restart and check the CPU after each.
Have you downloaded Process Explorer to see if any suspect items are running ?