i have a copy of 5.0.5, can i just delete 5.1 and drop 5.0.5. back in?
No. You'll create bigger computer problems if you write over Safari 5.1 with 5.0.5. System files have changed with Safari 5.1, which will be unreadable by v 5.0.5.
I'm not sure or confident about the Time Machine approach mentioned by others here. IMO, the best approach is to reinstall Snow Leopard via your DVD Installer.
Once Snow Leopard is reinstalled, run the 10.6.7 Update Combo, (download this update to your computer before running the reinstall), which includes Safari 5.0.5.
After the Update Combo is finished, open Software Update in the Apple Menu and download and install 10.6.8, plus any other updates appearing there (other than Safari 5.1).
When finished, open Disk Utility found in your Applications>Utilities folder. HIghlight your HD on the left and select on the right panel "repair permissions". Once complete you ought to be good to go.