In Windows Task Manger under the processes tab, click show processes from all users.
See if one of them is svchost.exe using a lot of cpu cycles.
If it is, you'll need to do windows update which you won't be able to until you activate Windows.

My laptop had this issue, which was quite common until MS fixed it in the most recent windows update.
Svchost.exe was running high cpu usage and taking up over 2gb of ram at idle.