Starting Hamachi automatically from a reboot or power-cycle:

Depending on your security needs, you can also setup the computer to automatically startup
the Hamachi free version by allowing the computer to boot up XP and login automatically. 
This way, Hamachi will get automatically started on a reboot, even though it's running 
as an application - not as a service. If the power fails frequently, or you need to reboot
your computer remotely, then this technique allows the computer to restart and automatically
restart and reconnect using Hamachi. Otherwise, you'll have to manually go to the remote
computer and login in order to restart Hamachi.
This technique works for a non-domain environment. 
You'll want to evaluate the security risks of enabling autologon. For my use, it works just fine. 

The technique described below is an alternative to running Hamachi as a service. 
The free version of Hamachi does not support running as a service. 
Note that the premium version of Hamachi does support running as a service.
It is possible to use the srvany or winserv programs to start the free version
of Hamachi as a service, but it is somewhat complicated, requires registry edits, and doesn't seem
to always work.

Steps to setting up Hamachi (free version) to automatically run as an application (not as a service) on a reboot or powercycle:

1) At a command prompt, type "control userpasswords2" and press Enter to open the Windows 2000-style User Accounts application. 
2) On the Users tab, clear the Users Must Enter A User Name And Password To Use This Computer check box and then click OK. 
3) In the Automatically Log On dialog box that appears, type the user name and password for the account tht 
you want to be logged on each time your computer is rebooted. 

Since you most likely don't want to leave the computer vulnerable with the automatic login, 
then do the following to automatically lock the computer after the auto-logon: 

1) Use Notepad to create a file called LockWorkStation.bat 
This file has the following 2 lines: 
rundll32 user32.dll,LockWorkStation 
exit 
2) Open the Group Policy editor - click Start, click Run, type gpedit.msc, and then click OK. 
3) In the console tree, click Local Computer Policy > Computer Configuration > Windows Settings > Scripts (Startup/Shutdown) 
4) In the details pane, double-click Startup. 
5) In the Startup Properties dialog box, click Add. 
6) In the Add a Script dialog box, browse to the LockWorkStation.bat file that you created earlier, and then click OK. 

So now when the computer is remotely rebooted, or reboots because of a power-cycle, 
it will automatically logon to the account of your choosing (starting Hamachi and whatever else you want started) 
and immediately locks the console & requires your password to unlock it. 

You can also keep the normal XP screensaver password enabled to lock the computer after so many minutes of inactivity.