Here's what I would do now.
1. Move the Steamapps subfolder somewhere safe.
2. go into control panel -> programs & featutres, and uninstall steam.
3. Reboot, then reinstall steam
4. Start it up - since your steamapps folder won't be there, you'll have to download games again. JUST download one - pick a small one.
5. test it, see if it works.
6. shut down steam (right click the icon, select Exit Steam)
7. Copy your steamapps folder back into your steam folder
8. now see if your games work.
Now, everything works okay after step 5, and stops working after step 8, we know something in your steamapps folder is messing things up. In which case, you can just delete it and start redownloading games again.
If it still doesn't work right after step 5, your problem isn't one that the usual tricks will solve. You'll need to contact steam support.