I had exactly the same problem.

• Removing the standard users, and making new ones, didn't work.
• Error-checking, didn't work.
• Kaspersky antivirus full scan, didn't work.
• Reinstalling windows, didn't work. I installed the free Ad-Aware Internet security by Lavasoft company. I made a full scan. It found some malware and I had to restart the PC. Since then the problem is completely solved.

I had exactly the same problem:

• Removed the standard users and made new ones; didn't work.
• Error-checking; didn't work.
• Kaspersky Antivirus full scan; didn't work.
• Reinstalling Windows; didn't work.

I installed the free Ad-Aware Internet Security application by Lavasoft and ran a full scan. It found some malware - after restarting the PC the problem was completely gone.