quote from BOINC website:
BOINC can be installed in any of several modes:
Single-user installation This is the recommended mode. BOINC will run while you (the installing user) are logged in. BOINC is listed in the Start menu of you,but not other users. The 'Show graphics' command in the BOINC managerworks only for you. The BOINC screensaver shows applicationgraphics only for you (other users can run the screensaver but will see textual information only).
Shared installation BOINC runs whenever any user is logged in. BOINC is listed in the Start menu of all users. While BOINC is running, it runs as a particular user (either the first user to log in, or the first to run BOINC). The 'Show graphics' command in the BOINC manager works only for this user. The BOINC screensaver shows application graphics only for this user (other users can run the screensaver but will see textual information only).
Service installation BOINC runs all the time (even when no one is logged in). BOINC is listed in the Start menu of the installing user, but not other users. The 'Show graphics' command in the BOINC manager will not work for any user. The BOINC screensaver will only show textual information.
Did you install as a Service?
If you right mouse click on the XP desktop, select 'Properties', then selct the 'Screen Saver" tab. BOINC should be listed in the drop down menu.
You could just reinstall the version of BOINC that you have. Version 5.8.16 is still available for download if you do not have it anymore. You should not have any split CPID issues with reinstalling the same version of BOINC on the same computer.
OF Course, I could be all kinds of wrong here and I am sure somebody else will correct me