I'm no expert but would suggest you download Microsoft Security Essentials & Malwarebytes (the free version - you want the left arrow).
http://www.microsoft.com/en-gb/security/pc-security/mse.aspx
http://www.malwarebytes.org/products/malwarebytes_free/
When you've downloaded them, find the "Update" tab & do so.
Now run them in "Full Scan" mode (not together) - they will almost certainly take a while to complete.
Please don't download/buy anything that says "we'll clean up your registry", or some such claim - they're taking you for a ride - registry issues are almost never going to be the cause of a slow computer. No-one should be paying for "security", as the real experts on AB have stated in the past.
If you leave your PC on 24/7, sometimes a router reset can help, as can clearing your browser cache (can't hurt anyway).
I hope this has been of some help & that I've not steered you in the wrong direction.