I'm supposing you're fully patched, especially since I personally hate it when the first answer to a problem that I'm having is 'are you fully patched?'
So, first try cleaning your CD. If you've done a full install, try switching the CDs around to see if it'll help out.
Reinstall your patches, fixpack, and then all your mods in the correct order. There might be something wrong with the ascension mod.