After installing Windows with Boot Camp slows down OS X performance

On my intel-mac 20", I install bootcamp and partition my hd with boot camp assistant. Then I install windows xp sp2. Windows works almost perfectly like a non-apple pc but my mac os x performance gets awful. It looks like it's derieved by the partitioning. Because os x boots very slow and i hear too much hd sound. And launching a program takes longer time than normal. I want to use both OSes with good performance.

-Isn't it a bit weird that win works smoothly and fast but os x is slow?

-Has anyone encountered that problem?

-Is it a solution to partition disk before installing OS X and then install OS X and XP? ( *sigh* i can't connect an internal hd cause this is an iMac. )

I tried too many times but result is always same. I only want win because i like games. In the other hand, mac is always better. I was googleing for so much time but i couldn't see anything same with my problem so i'm posting it here. Waiting for your help.

I recently re-installed OS X to fasten my mac. So there's no bootcamp now but i remember it was more than half of the hd(it's 250GB) for the OS X ( xp partition was about 80-100 i think ). The weird thing is i get poor os x performance even i re-partition my disk to one piece.

Edit: How much hd space for xp do you recommend?

Edit 2: One more problem. When both XP and OS X is installed, some files i put on desktop disappear but when i search the filename at spotlight, it appears again. Often, after pressing empty trash button, trash still looks full. I open trash window and click on the files, they suddenly disappear and trash empties. This one is really really weird.