How much RAM installed?
How large is Windows partition...and how much free space on that partition?
Neither of your suggested causes...is probably correct, IMO.
Please download MiniToolBox , save it to your desktop and run it.
Checkmark the following checkboxes:
List last 10 Event Viewer log
List Installed Programs
List Users, Partitions and Memory size.
Click Go and paste the content into your next post.
Also...please Publish a Snapshot using Speccy - http://www.bleepingcomputer.com/forums/topic323892.html/page__p__1797792#entry1797792 , taking care to post the link.