Quitting apps and freezing mouse both point towards
software issues, as in corrupted preferences or outdated drivers...easy fixes
Errors writing to DVD could be either software or hardware related, but since the drive will PLAY dvd's ok, I would look at:
A) amount of free space available on the boot drive, and
B) Try different brands of disks, and
C) Amount of physical ram installed
With a 20 gig HD and the average install of OS, apps, utilities ect, you are probably getting a little tight on space......
I would also recommend updating to 10.4, which has even better support and improved frameworks for disk burning