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
Depending on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the suspect file, or ask you for a desired action.
More information on the scanning and removal options available in your F-Secure product can be found in the Help Center.
You may also refer to the Knowledge Base on the F-Secure Community site for more information.