While you may have lotsof of space on your HDD, you may not have sufficient memory, or RAM, installed on your computer. If a program wants 220MB of memory to run and you only have 200MB available, you will see a window with an error message pop up when you try to open it. The error may say more memory is required or that you require more virtual memory.
There are two techniques to resolve this problem. The simplest technique is to quit all open applications and try running the program again. By quitting other programs, you free up the memory they were using. This will enhance your computer's available memory; probably enough to run the program that before would not open. If the program will also not run, you will require adding more RAM to your computer. For example, if you have 256MB of RAM, you could double your memory by adding another 256MB RAM chip. You could also add a 512MB chip, which would give you a total of 768MB of memory. Before purchasing more memory for your machine, consult your computer's manual to find out what type and how much RAM it supports.