The HPS or Hanta virus activates on Saturdays and will flip bitmap files, including the Windows 95 logo.

Virus — blank screen, unusual messages, unusual behavior

What to Do
1) Use anti-virus software to remove the virus.
2) If your data is not accessible after removal, do not run a commercial utility. Contact DriveSavers for professional recovery.

1)Software may remove a virus, but it does not always reverse its damaging effects on your data, which may remain inaccessible.
2) Virus damage can be severe enough that a commercial utility can cause further damage and make data recovery more difficult.