Eggheads advice is the best solution and it's what I did many moons ago when I had your very problem, but I have something else to add.

Sometimes a website will size the browser window automatically. If this is the last IE window you close, then it will remain as the default size and you will have to reapply egg's solution.

I found a program which will automatically maximise your IE windows everytime one is opened. It's called IE Maximiser (funny bout that) and can be found here: http://www.jiisoft.com/iemaximizer/index.htm

It's freeware too