CPAV is not a virus, but an antivirus program. If F-Secure anti-virus products says that a program has been modified by Central Point Antivirus, it means that CPAV has added a self-checking module to the end of the program.
When a program modified in this way is executed, the added code will first check that the length of the program is not changed, and allows the program to execute only if this is not the case.
F-Secure anti-virus products will detect programs modified in this way because the added code resembles virus code closely, and also because several programs (including F-Secure anti-virus products) will not run after a modification like this.
The added code can be removed by CPAV itself.