Check your PATH environment variable. The default installation of Solaris does not include the correct PATH for newly created users. If you use bash, which I usually do for users, you'll need to create a .bash_login file.
Example:
export PATH="/bin://usr/bin:/usr/java/bin:/usr/perl5/bin:/usr/X/bin:/usr/X11/bin:/usr/ X11R6/bin:/ust/dt/bin:/usr/openwin/bin:/usr/sfw/bin:/usr/staroffice7/program"
Logout and back in, or type source .bash_login and type which xscreensaver.
Check to make sure the GLX extensions are in place for the nVidia graphics by typing glxinfo after exporting the above sample PATH I wrote. The nVidia drivers are needed for GLX support on Solaris x86, and in my opinion the x86 GL implementation on Solaris not as mature as SPARC's.
The patches fail to install if permissions are wrong on /var/spool directories, /tmp and /var/tmp, or if you are trying to use RBAC permissions with Sun Update Manager. Use root as the documentation says if you are somehow running it as a user, which it shouldn't be able to do.
I had a problem on my Ultra 80 where the background cacoa daemon was not functioning property. This caused Java programs, notably Sun Update Manager to fail to start, freezing the X11 window, idling there of course, and the only way to fix it was a reinstall. The installation was corrupted by scratched media and an interrupted smf build. Make sure that svcs -vx doesn't output any degraded services, make sure that java -version works, and make sure that when ran from the console, updatemanager doesn't spit out any errors, that may help in debugging this issue. You might also want to read Sun's docs on patchpro, which handles the actual bare-metal OS patches behind the scene, it may be malfunctioning from something as simple as a missing symlink, environment variable problem, or missing installation.
Sorry to be a bit vague on the solution, but check /var/adm logs also for any extra error information that might had made it to the system logs.