Like I said, check your processes - see what the space hog is. It may be something running through svchost (which then becomes a PITA to find).
It would not hurt to start here (Support | Dell US
) and check with Dell chat or phone support if you still have any coverage.
Check for new drivers; the video driver would be the first I'd go after if there is one - but update things one at a time.
There are quite a few viruses (trojans especially) that hide from task manager and Glary. Rebooting into safe mode and running ComboFix may be a really good direction to go as well. You are at the lower end of RAM for a business laptop but for a home user it should be enough, assuming no malware or bloatware is maxing the resources out.
(If you are particularly savvy you could check the size of your swap file. I have yet to pursue it on an 8.1 install, but had to under XP, Vista, and 7.)