My computer has been effected with many Trojans and it changed the display properties when i want to change the background of desktop. After it's been efftected it also changed the background of desktop by itself. How can i change it back to normal settings?

Assuming you've already identified and removed all the intruders on your system, you could perform a refresh install of Windows to fix any damage that has resulted.

http://support.microsoft.com/default...&Product=winxp

Note: you will most likely need to reinstall Windows updates afterwards.

Wow, I enjoyed that answer you got. It didn't solve anything.
Try this. Go into the registry and look for
[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\
System]

Here if you have a key that says "Hide Settings" of any kind and it has
a 1 at the end of the number " will look like this "
ReG_Dword 0x00000000001 <----see the 1?
then you need to change it to " 0 "
it will look like this
Reg_Dword 0x00000000000 <-----see the 0?
I found this fix after I read the comment left to you by the kind person who would rather you reinstall windows. The answer isn't always hard.

I did remove the spyware that had caused this so in the event that you successfully make the change and it comes back you may be still infected with the spyware. Mine was vx2
MEAUXMONEY