Most likely an automatic MS update has changed the settings of the computer, or corrupted the system files. This happens quite often. From what you describe, it's not a virus.
click all programs
click system tools
click system restore
check the box that says: restore my system to an earlier date
select a bold date on the calendar, a date before the problem started
follow directions to restore system
System restore does not affect your personal files, and is reversible, you can go forward or back at any time.
It merely removes the offending changes to the software and system files of the computer.
when restore has completed, restart computer
if problem was not fixed, if system would not restore, repeat above using an even earlier date
system will be set back to a time before the MS update corrupted the settings or system files.