The XA1 virus overwrites the beginning of files with itself and stores the original code at the end of the file. On April 1st, a part of the virus will activate - overwriting the boot sector with code that will cause the computer to "hang" on next boot-up. The virus will also activate on December 21st and stay active until the end of the year. It will then display a Christmas tree, and the text:
Und er lebt doch noch: Der Tannenbaum! Frohe Weihnachten
Allow F-Secure Anti-Virus to disinfect the relevant files.
For more general information on disinfection, please see Removal Instructions.