Alveranga_ Do you have Steam's family sharing enabled? If so, please disable it.
Otherwise, please provide a dxdiag. Hit Windows key-R, enter dxdiag in the run box, wait for the scan to finish, click "Save all information," and save the file to your desktop. From there, you can attach it to a reply using the paper clip (Attachment) icon included with the other formatting buttons.
chloehit It sounds like you've established the problem is something in your Mods folder, which means you'll need to sort through the contents of that folder. For further help, please post in the Mods forum:
The Sims 4 Mods & Custom Content | EA Forums