It's a file on your computer too big to store in the place on the hard drive where it was originally created. So it gets cut up and spread out into various places. Naturally, it will take your drive longer to use the file because of the need to do multiple operations to open the file.
Periodically it's good to "defrag" the hard drive. If you've never done it, it may take a long time the first time. But if you do it regularly, such as once a month, it's a lot faster.
In case you are not aware, in Windows XP, you'll find the Disk Defragmenter under All Programs - Accessories - System Tools
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What is a fragmented file what does it mean