Umm, well, my guess would be that you're trying to delete things that you really don't want to delete. Most of the files that you don't have 'sufficient privileges' to delete are system files that your computer needs to operate.
What exactly is it you're trying to get rid of? If you're really concerned with getting as much as possible out of your harddrive, then it may be worth it to format your harddrive and cleanly install the entire operating system without extra things you don't need, such as languages you don't read/speak, printer drivers you'll never need, and any applications that you don't use. Just make sure that you have everything you need backed up somewhere first!
If you're trying to delete files that you know you should be able to delete, but it won't let you, then click on the file and hit command-i to get the file info. Then look under the 'Ownership and Permissions' tab and make sure it says that you're allowed to read and write the file.
Tim
