I have the same problem and suspect it's not uncommon. Older ATI AGP hotfix drivers did work, although intermittently. With the 11.x (all that's available now, apparently, on the AMD driver site), I can't get the driver installed at all. Help?
The problem apparently was that only some older drivers are stable on my machine. I found an archive of drivers at the Sapphire site that included version 10.10 (someone recommended 10.10 as stable). So far it's been stable. I did run the Catalyst uninstall and Driver Sweeper first. Why AMD doesn't archive these drivers I don't know.
Sapphire's Catalyst AGP Hotfix archive: http://www.sapphiretech.com/presentation/downloads/download_index_01.aspx?psn=0006