Another tip to try Arriba is...
Go to your control panel and open your "power options"
click on the "advanced" tab
ensure the "prompt for password when computer resumes from standby" choice is unchecked
I have had this problem with PC's in the past and it has been due to a corrupt Virtual Memory (something that a system restore wont fix).
Go into control panel
System
Advanced
Performance - Settings
then click on the advanced tab
Virtual Memory - set that to no pagefile
Then reboot
then when its rebooted put the pagefile back again set it to 2048 or system manage.
See what happens then, also a very good 15 day trial software that will check out your whole system......
http://www.iolo.com/downloads.aspx system mechanic pro, I use it regularly it will check your whole system out for problems.