Missing Explorer, but not

Upgraded to Explorer 8, which then started having problems and would close. Add-ons were suppose to be the problem. So ran explorer without them (don't actually know how I did that, because later I couldn't figure how to reverse the process and run explorer with them).

Now internet Explorer doesn't work except to connect with my homepage which happens to be a University site. I can navigate within that site, but nowhere else.

Have run virus searches. Comes up clean. Symantec (which is required for the University); Superantispyware; and Spybot.

Can't uninstall Explorer 8 or replace it with Explorer 7.

I installed Firefox which will not run -- "the proxy server is refusing connections" Incidentally where would I to to set that proxy on or off?

Also tried to revert to an earlier date. Even though I have many days to go back to -- there is an error that doesnot allow it.

Any suggestions?