This could mean you havent got enough memory left (computer has to much on. )Mines being doning this for ages but the best thing to do if it carrys on is to backup any files you want to save and do a system restore. click start , search, files or folders type in sysytem restore press go. On the right hand side of page you will get a list come up click on the icon system restore and restore to the last time you got computer or had your windows package put on. it means your have to reload all your applications but it's worth it.
One main thing to do once done is to make sure you do a windows update. (START, WINDOWS UPDATE, UPDATE AND REINSTALL UPDATES.) MUST DO ALL THE CRITICAL ITEMS.
GOOD LUCK!!!