I just have the sa user to log into a sql 2005 express server. the service is running as networkservice. is there a way to change it to localsystem with sql server management studio or any other software connecting from the network. i have no other login to the server
Unfortunately (or perhaps fortunately), no, you cannot.
You could in theory gain access to the computer via the xp_cmdshell SP, which you have permission to run when you use a SQL login that has the sysadmin server role.
However, the NetworkService account does not have access to alter service configuration (including services it is running itself).