It's a hidden partiton, and there should actually be two hidden partitions. One is a restore partition, which you can use to restore your computer to the state it was shipped in. This should be the larger of the two. The smaller one should be a partition which just contains diagnostic tools and stuff. Basically, I think it's okay to delete both of them, as all the diagnostic tools can be found on the website, I believe, and also, if you reinstall anyway, that means you have a CD, and you don't need a restore partition. The restore partion would get rid of all the files, as would reformatting, so it's useless if you have a CD and you are comfortable with reformatting and reinstalling the OS.