While many CD burners have suitable buffer amounts that should allow you to multitask while burning, this is not recommended. Extra draws on the system resources can reduce the chance for a successful burn, as well as causing errors in otherwise perfect CDs.
If you are running Windows 9x or ME, it might be best if you reboot before beginning a burn. Many times the system will not release memory that was in use by processes that you have used while the computer was on, and can cut severely into available system resources. 70% or higher is recommended before beginning a burn.
So, when burning a CD, the ideal condition is a PC under light use (preferably very little other software running).