I’ve deleted some Keychain certificates and now Internet browsing is almost impossible

Background

A few months ago, for reasons I am unaware of, Safari started to ask me for a password when I first opened it, and then periodically after that (about once every 3 new tabs I opened). I found that to get rid of this message I could either enter the correct password or simply click "cancel" twice.

This problem is documented here (I'm not the OP on these):
https://discussions.apple.com/thread/2537287?start=0&tstart=0 and here:
http://b.rthr.me/wp/?p=356

As one can imagine, this was very annoying and lead me to take action. I followed every piece of advice that I could (set date and time to automatic, reset Safari, try Chrome etc) and the problem still persisted.

However, I read somewhere that if I opened Keychain Access and deleted everything (!) then it would kind of 'reset' itself and work as normal again. Reluctantly, I tried this. It did not work… it made things worse.

The Problems I Have Now

• Every webpage I go on, Safari brings down a notification saying "Safari can't identify the identity of the website website-name-here.com". If I click continue, I have to enter a password and the page loads (partially – see below). If I click cancel, the page will not load.

• For some websites, some of the content is blocked. For example, on Google Maps, each and every 'map tile' just appears as a little blue square with a question mark in it. A question mark like this also appears, for example, where the 'user icon' should be (next to the username) of a user on this site (but not for every single user). This 'blocking effect' also means Netflix won't play anything that I've tried.

Additional Notes

• I'm on a Macbook 3,1 (from 2007) with a broken superdrive and I'm running Mac OS X 10.5.8.

• It seems as if I can't even upload images to help describe the problem. If I try to upload a screenshot, I get "security reasons, framing is not allowed".