Couple options:
Does the xbox still boot games? If so, boot up the exploit, and install UXE Complete from "the usual places". Your xbox will be fine. UnleashX has memory card/save management. To get your dash back, you could cold-boot a live-enabled game (SC should work, actually), connect the xbox to a network, and select LIVE, the latest dashboard will install.

Alternately,
get Slayers AutoInstaller disc, and extract the contents with QWIX. The original dash files are in side a c:\ folder somewhere.
BTW,
which softmod did you install? The SC exploit is just an access point. Did you use Krayzie's UXE, NDure, a crappy old font exploit, what?