On a Dell, it's usually ctrl-F11, as told in (for example) http://en.kioskea.net/forum/affich-97164-restore-factory-settings-on-dell-inspiron.
But it could be different on your Dimension e310. Did you look at http://support.dell.com/support/index.aspx or in your user manual?
I have a Dell Dimension e310 with XP Media Center. I don't have the recovery disc and I would like to restore it like when it was new. I've read that it is possible to reformat the Hard Drive by pressing F11 when rebooting. Is this true?