Hi Louri,
Take a step back, and give us more detailed information about your computer, make =HP, Model = Presario RP, CPU, HDD's, how set up, size, how much free space left, what you mean by a pop-up etc. OS = Win-XP SP3? Then exactly what has happened and if it is acting unusually, other than the disk full msg.
Could be just a good cleaning of your temp and Temp Internet files may be the answer.
Do you empty your temp internet files on a regular basis? Also do you by chance have CCleaner on your computer.
That may give the group a better insight as to how to solve this problem