I eventually resolved the problem a week ago. I engaged the services of an IT professional. At the start button he ran msconfig and disabled non-essential programs at start up, such as Skype, Nero etc that use a lot of resources. He also loaded a memory optimiser. I can now run macro and open several workbooks without encountering any problems. I have tested this for a week now
Well done - freeing up all the resources you have will make much more available and reduce the likelihood that you will hit the limit again. Msconfig is the standard method for managing startup apps but another MS product, autoruns, free from the link below is a much more powerful and comprehensive manager which you might find useful for monitoring startups:
You mentioned Nero. If, you are running Windows 7, there is no need for Nero or any burning software since 7 does everything natively & there is no longer need for adding burning software. 7 will, even burn ISOs by itself & direct burn to both CDs & DVDs.