You are unintentionally shutting down important system processes. When this occurs, the system goes into distress and shuts itself down. When it reboots it has re-enabled the processes you disabled, which allows it to function normally once again. Until you terminate them once again, which causes the system to reboot to re-enable these items again.
System Processes running in the "owner" User account are not necessarily not critical processes. This simply means they are running in the account currently logged on.
The only time it is OK to terminate such "owner" processes is when the account used is a Limited User account, not an Administrator account. An Administrator account can and does often have critical system processes running in that context.
The reason some processes running in the context of a Limited User account is due to the fact that nothing critical is shown on the processes tab of the Windows Task Manager which a Limited User could shut down. If it is listed and the user attempted to shut down a critical or important System process the user would receive an Access Denied message.
Windows Vista has basically done away with the Administrator and Limited User account system. While the account system in Vista resembles the XP system it isn't similar at all. Every account is a "Limited User" account, but those users with Administrator privileges are allowed to make system wide changes or configurations. Which is why the Access Control causes the system to request elevation and/or an Administrator credentials.
A very solid sign that some of these processes you are shutting down are critical is the fact the system continually reboots itself after you have disabled them. This allows the system to re-enable these critical processes.
A good method of determining if a process is from a application or third party service which is not critical is to check the processes name. Anything you can easily recognize as an application, or third party service, such as wlmail.exe, or Windows Live Mail would be safe to terminate. Another example is DrgToDisc.exe, which is Roxio Drag To Disc application.
If you don't recognize the name, you can use Search to locate the executable and then righ click select Properties, click the Version tab and read what it says. If any of them say Windows Operating System leave it alone. scvhost.exe, rundll32.exe, ctfmon.exe, csrss.exe and many others are critical system processes. ctfmon runs in the context of my User Name, but it is still critical and would cause the system to reboot.
Also, a processes doesn't have to say Windows Operating System in order to be a critical process. If you have any doubt after performing a Search, Google the executable and read what it says Online about the process.
There is another option for creating environments to play computer games in. You can use the option to create hardware profiles where you can disable the none essential services. Here is a website which will help you do this. Please remember to check any item in the task manager which is not listed on this website prior to disabling it because I found some important third party services were not listed when I was creating my hardware profiles:
http://www.blackviper.com/
The Black Viper website is a popular site which has "manuals" on different topics, but the major one is on creating hardware profiles which allows you to create an environment which has minimal services for game playing. It really rocks!
As I mentioned before, if you see a service in your service console which is not mentioned at Black Viper, just Google it and research it before disabling it in a hardware profile.
Good luck and much fun with your games, please have a nice day.
*edit* Yeah, ctfmon is a critical system process. I didn't look at too many of the others because I saw that and went "Ah ha!". That would cause the system to reboot if it were disabled.
Anyway, look into creating a hardware profile where you disable all but the essential and critical processes and services. Good luck.