That all depends on several factors i.e. CPU speed, RAM, FSB, and the like. Tell us what your have onboard and exactly what kind of multitasking you would like to do. Rule of thumb - for the least amount of problems, do NOT multitask while burning anything.
You can always check CPU usage through task manager and see what percentage of your systems resources are being used.
I do a lot of other things while I'm burning but as MarkRacer said, it is very dependent on your PC. The newer DVD softwares and newer PC's are much more tolerant of muti-tasking.