Computer and Internet very slow

Dear All,
At my office computers, one of them is very extremes slow. When i tried to launch any applications especially When i open Firefox and sending birthday card,it is very slow of sending and hang up..,
and it take several minutes to make it as normal.

I tried to run the Avast scan, i didn't see any of detections even Microsoft Security Essential.
But when i run Malwarebyte, it was detected some viruses and i deleted it than restart,
The computer seem the same slowing after restart.
So i tried to run BitDefender Live CD scan, i got 10 detected thread with 28 containing in those detection..Most of them are *.ost files.
When i clean it, after that restart machine, it's seem a few better.

But when i launch Firefox, and try to send birthday card from website, it seem spin around and take around one minute to complete sending.. The thing just better than before..

As your ideas, what is that problem?

Best regards,