I installed many applications on my n900. but now i want to return my phone as manufacturer programing to make it clean and to reinstall applications that i found it important. But I want to keep backup for the messages and contacts only. can you help me please with this issue?

You can use the Backup application in the application menu.
Make a backup with it for whatever you want, and then save the backup file to your PC.
After that, reflash the eMMC and the firmware of your device. (You can find walkthroughs in the wiki, and also this forum.)
It the reflash is complete, copy your backup back to the device and you're good to go.

Or, you can just uninstall the unwanted applications.

Please can you explain step by step how should I make re flashing for my N900?
I read many things bur i didn't get the conclusion.

It's all here.

If you have a specific question about it, don't hesitate to ask.

Sorry to disturb you again but really I need your help.
I have windows Xp system. so I entered the page of downloading flasher. and I downloaded "maemo_flasher-3.5_2.5.2.2.exe" on my laptop.
After that I entered the of downloading firmware and I don't know what I should download since there are many items.
is the downloaded "maemo_flasher-3.5_2.5.2.2.exe" is the right one? and what I should choose for the firmware?
once you will help me by these questions, i will continue and i will ask you again for any problem i face when i will make reflashing for my N900. ;-) thanks alot for you