Vista - browser variable

in the past, i had a virus and clean it.
now, if i used any email client, the url link doesn't open my web browser
internet explorer. I have tried a different web browser(firefox) and the URL
link works well. But i want to use internet explorer as my default browser. i
have tried many things to fix that bug, but it is still there. Also, i use
BOINC, it is a program that is running in background and he works well. In
the past, the url link works fine, now, it says that the BROWSER variable as
to be set. with firefox no problem. With internet explorer he does not find
the BROWSER variable that executes Internet Explorer.
I have tried to setup firefox first as my default browser, it works.
everything works fine. when i switch to Internet Explorer, the url link of
any programs who calls internet explorer doesn't work. I have to do a cut and
paste in any programs and paste it in internet explorer to make it works,
also the url inside IE works, it's only when we call IE that is the problem.
How i can repair internet explorer to be able called by other programs with
the url link?
ty