If you're not an IT specialist, using Internet Explorer is very unsafe to your computer in any case, especially older versions. You are a lot safer with Firefox. Partly because Firefox actually tries to keep you safe (IE instead simply transfers all responsibility for safety onto you!)
And partly because Internet Explorer is tightly integrated into the entire Windows operating system. If you hack into that browser, you have hacked the entire computer. Firefox is less integrated that way.
But I really don't recommend Vista for anyone in its current form. Just say no. Stay with Windows XP, or buy a Mac.