i've deleted some Keychain certificates and now Internet browsing is almost impossible
Unfortunately, based on your comments, it looks like your only option is to start afresh.
* Backup everything first (use Time Machine or just clone it using SuperDuper or Carbon Copy Cloner)
* Do a fresh install of OS X Leopard. You can do it from an external SuperDrive or using another Mac that has a SuperDrive, with this one put in Target Disk Mode
* Run Software Update to bring the system up-to-date
* Use Migration Assistant (under the Applications->Utilities folder) to bring over your applications and data from the backup

Also see Reinstalling Mac OS X without a DVD Drive - Target Disk Mode