I can not find in application folder
conf.json to change client from full to lite.
Where is the configuration file can be found in Windows XP
When you say application folder, do you mean %LOCALAPPDATA%\byteball? That's where you should try looking. Having said that, you can not change a client from full to light and vice-versa. You have to reinstall and chose light.
When I tried to reinstall...
I've got error:

System: Windows XP 64 bit.
I can provide the necessary logs.
XP is not supported, 7 is minimum.