exactly how can I find out which MS explorer browser I'm using?

I need your help but it must be detailed as I know little about computer hardware/software.

Thank you.

By sheer luck I was able to reinstall the Vista Operating system when my home computer totally stopped operating. (this computer and OS is about 8 years old.

Of course I lost all the information I had previously stored.

I tried to enter my account in Bank of America but I got a message when I was on the bank's

web site that the Explorer browser could not access this account.

I tried a few times with the same result.

I then tried to access my Capital One credit card account and was successful.

But when trying to access my Discover Card account I got the same rejection,

of no access as I got when I tried the Bank of America bank account.

I'm guessing that I have an old version of the Explorer browser which is causing this problem.

But I don't know how to find out which version I have.

Or how to up date to a newer version of Explorer.

I want to stay with MS Explorer to minimize my confusion in jumping to a new browser like firefox, etc.

Thank you VERY MUCH for any and all help