Backing up site
a) just to do the whole site backup?
b) "backup" the data from your old install so that you can then import it into the new install?
For a), package up (zip, tar maybe) the whole site root (e.g. public_html) and take a database dump, those are enough for you to bring back your site.
For b), it's quite a lot trickier issue and if you have a quite small site, I guess it would be easier just to manually import the data there after the clean install.
Antti / Mainio