I think you must be using Window Vista, becuz this always happen in Vista.
Here is what you can do ... you have to install your self as the administrator.
1. Click start windows button
2. click programs
3. click accessories
4. Right Click command prompt and select run as Administrator
5. click continue
6. type in with out quotes "net user administrator /active:yes"
7. enter
8. restart computer
Hopefully, this fixes one very annoying problem with Windows Vista. I did notice that upgrading to SP1 helped to reduce the occurrence of this error, so if you do not have SP1 installed yet, make sure to go to Windows Update. Enjoy!