With large jobs like DVD’s, which may need 4 gigabytes or more of drive space to cache the files, it may end up taking more space than your drive may have.
If Windows XP Virtual Memory is activated, which it is by default, physical RAM may not be as much of an issue as it might otherwise be. You can burn DVD’s if you have 512 meg of RAM for instance, cause it caches the rest to the hard drive and processes it in chunks.
If your hard drive is not properly tracking due to lost clusters or something, it can get a bit confused, even if you technically do have enough hard drive space. If the file system gets confused, doing a Disk Error Check and Fix can help get it back on track.
Sometimes huge temp files are left on the disk and take up space you might otherwise have available for other things. There are programs like CCleaner that can help with that, if you are unfamiliar with doing it manually.