Hi and Welcome to PSG
You don't really have enough RAM installed for PS, 8GB would be the bare minimum that I would recommend, also don't let PS use all of the available RAM. This should be set to about 70% by letting PS use all available RAM there will be nothing left for other apps and the OS
You have to remember that Windows itself will be using about 2GB and up of memory and PS is really RAM hungry and I mean hungry!
The only option you really have is to install more RAM, max out what your system can support
You can use this scanning tool to check what RAM you have and what your system can support
http://eu.crucial.com/eur/en/systemscanner
Take a look here
https://www.slrlounge.com/best-computer-specs-photoshop-computer-par/
P.S.
Are you using a 32bit version of Photoshop, if you install more RAM you would be better installing a 64bit version that's providing you have a 64bit version of Windows installed
If you don't have a 64bit version of Windows then there's no point installing more RAM as Windows 32bit is limited to 4GB of RAM.[/QUOTE
i have 64 bit version , sooo i thinki should upgrade whole pc since its 5 years old and rams are ddr2 800
i thought maybe another trick or option would solve this , thank you...