I had the same problem after upgradeing from Vista32 bit to Vista64 bit. Here's what I did to fix mine;
1. Click start
2. Type Run MsConfig
3. On the System Config Window Click on the Services Tag
4. Click on the "hide all Microsoft Services" option.
5. Click on the Start Up Tab at the top.
6. Scroll down to the Windows Defender box and Uncheck it.
7. Go back to the Services Tab and Uncheck "hide all Microsoft Services".
8. Click on Apply, and OK.
9. Restart Computer
This took care of my problems. Hope it helps you too!