When you install a distro, it installs the kernel, and all the software. If you have all your user data on a separate partition from your programs/kernel/etc, then you are fine, just make sure that your user partition is not erased during the new install.
Generally, all your user files are stored in /home/username (if you have a webserver, the webpage files may be stored somewhere else, like /var/www/htdocs or /home/httpd). Just backup all these files, and then restore them after you are done installing a new distro. Depending on how much stuff you have, you might just be able to dump it all on 1 or 2 CD-Rs.
If you have a lot of data (several gigs), so that it is not feasable to backup to CD-Rs, and everything is on the same partition, what you can do is use somethig like parted http://www.gnu.org/software/parted/parted.html
or Partition Magic to split your partition into two, and move all of your user data you want to keep to the second partition, while leaving all the software on the first. Then you would be free to erase the first, and install a new distro there.