prefetch files

hi,
I have windows XP pro, AMD athlon XP 1800+ 1.53GHz,352 MB of RAM.
I was just wondering what kind of files are contained in the prefetch folders. Somebody earlier mentioned here that those files can slow the system down. He suggested that those files have to be cleared on the regular basis in order to maintain the speed of your system. I was afraid to clear these files. I therefore decided i should first ask any one who understand the contents of this folder.
Any suggestion or comments will be highly appreciated.

Thanks.