Microsoft Corp. says Windows XP, the new operating system it sent to computer makers on Friday, will be more stable, secure and feature-rich than any previous version of the software giant's flagship product. But Windows XP is also more demanding of computers, meaning many consumers who bought machines more than a couple of years ago will not be able to run the powerful new software. Microsoft recommends 128 megabytes of RAM. RAM, or random access memory, stores open programs to make them run faster. The more RAM a computer has, the more programs it can run at the same time. Microsoft says XP can function with just 64 megabytes, but that some features may not work. Windows XP will also need lots of hard-disk space -- 1.5 gigabytes worth. A small hard-drive or one that is fully loaded with other software, will not be able to hold XP. And although Microsoft says XP can run on a 233-megahertz processor, it recommends a chip running at speeds of at least 300 megahertz. Slower speeds will make XP run sluggishly. Most computers bought in the past two years will easily meet those requirements. Others can be brought up to snuff after shelling out a few dozen dollars.